feat: add file-based admin settings panel at /admin (#866)

* feat: add file-based admin settings panel at /admin

Settings saved in the panel are written to data/settings.json and
overlaid onto process.env, taking precedence over environment
variables and applying immediately without restart. Enable by setting
ADMIN_PASSWORD; on serverless platforms without persistent disk the
panel degrades to read-only.

* polish: admin panel UI improvements

- Provider logos in credential rows (shared ProviderLogo component,
  extracted from model-config-dialog)
- Scroll-spy active state in the sidebar nav
- Green success state in the save bar that clears after a few seconds
- Wider content column (max-w-6xl) for less wasted space on desktop

* polish: admin panel section toggles and reorder

- Move Quota & Rate Limits to the end of the settings page
- Add enable switches to Observability and Quota sections; default off
  with fields grayed out, auto-on when any field is already configured

* polish: make section enable switch more visible

Wrap the switch in a labeled pill ('Enabled'/'Disabled') with border
and background so the off state is clearly visible.

* refactor: derive admin registry from PROVIDER_INFO, simplify page state

- Provider options, labels, and base-URL placeholders now come from
  PROVIDER_INFO instead of hand-copied lists (fixes SiliconFlow .com/.cn
  placeholder drift; panel names now match the model-config dialog)
- Replace free-text subgroup strings + SUBGROUP_PROVIDERS reverse map
  with a typed provider field on SettingDef
- Precompute SETTINGS_BY_GROUP and PROVIDER_SUBGROUPS at module level
- Merge justSaved into saveMessage, drop unused mainRef, hoist
  fetchSettings out of the component, dedupe savedText logic
- Serialize from SETTINGS_REGISTRY directly; json validators in a map
  instead of a hardcoded key check
- Make allowPrivateUrls a function so ALLOW_PRIVATE_URLS edits in the
  admin panel apply without restart

* feat: graphical model management in admin panel

Replace the provider credential fields and raw AI_MODELS_CONFIG JSON
textarea with a Models section mirroring the in-app model settings UI:
provider instance list with logos, credential fields per provider type,
model add/remove with suggestions, per-model connectivity test, and a
default-provider star.

On save the server derives everything the runtime needs into
settings.json: credential env vars (with _2 suffixes for multiple
instances of one provider), AI_MODELS_CONFIG, and AI_PROVIDER/AI_MODEL
for the default. Secrets round-trip as masked markers and are never
sent back to the browser. The general settings registry now only
covers non-provider settings (generation, access, features,
observability, quota).

* fix: allow testing unsaved providers in admin panel

The test button previously looked up credentials by providerId in the
saved settings, so testing a newly added (unsaved) provider failed with
'Unknown provider or model'. The test endpoint now accepts the client's
current provider state; newly typed secrets are used as-is and masked
markers are resolved against the stored values, so testing works both
before and after saving.

* fix: merge env AI_MODELS_CONFIG with admin panel providers

Previously, saving in the admin panel wrote a complete AI_MODELS_CONFIG
into settings.json, which (by overlay precedence) replaced any config
from .env or ai-models.json — admins lost their env-configured models.

The panel no longer writes AI_MODELS_CONFIG. Instead its providers are
merged with the env baseline at read time in loadRawServerModelsConfig,
and panel credentials go to ADMIN_-prefixed env vars wired up via
apiKeyEnv/baseUrlEnv so they never shadow standard vars. Env-based
providers now appear read-only in the panel, name clashes are rejected,
and a panel default overrides the env default. data/ is now gitignored.

* fix: block global-credential providers already managed via env

Bedrock, Vertex AI, and Ollama credentials live in fixed env vars with
no apiKeyEnv redirection, so a panel instance of one of these would
silently override the credentials that env-configured models rely on.
The API now rejects saving such a provider when the env config already
uses that type, and the Add Provider dropdown disables it with a
'managed via env' note.

* fix: address admin panel review findings

- Security: test-model no longer resolves a stored secret when the
  request's baseUrl/provider differs from the stored entry, closing a
  path where a tampered baseUrl could exfiltrate a saved key
- Save failures are now visible: the save bar shows the error in red
  (was masked by the persistent 'Unsaved changes' text), and per-field
  validation errors from the settings API are surfaced under each field
- The Observability/Quota enable switch is now real: toggling off stages
  deletion of the group's saved values, and the toggle no longer snaps
  back to Enabled after saving
- Env provider's default star is hidden when a panel provider is the
  active default (no more double star)
- Clearing a credential field reverts to the stored value instead of
  silently deleting it; an explicit X button removes a stored secret
- Form inputs are disabled during an in-flight save

* refactor(admin): split 1549-line admin page into focused modules

Extract admin-shared.ts (types + fetch helper), setting-field.tsx
(registry-driven fields), and models-section.tsx (provider/model
manager) from page.tsx. Pure mechanical move, no behavior change.

* feat(admin): share credential fields with user dialog and localize panel

Extract ProviderCredentialsFields (display name + per-provider
credential inputs) used by both the user ModelConfigDialog and the
admin Models panel; secret input passed via renderSecret (plaintext
vs masked), test button via footer slot. Add full i18n for the admin
panel across en/zh/ja/zh-Hant, reusing modelConfig.* for shared parts.

* fix(admin): address Copilot review findings

- Reflect built-in defaults for boolean settings (ALLOW_PRIVATE_URLS
  defaults on) and allow clearing a saved boolean back to default,
  so the SSRF toggle matches actual runtime behavior.
- Harden JSON loading: filter settings values to strings only, and
  schema-validate stored ADMIN_PROVIDERS entries, dropping malformed
  ones instead of letting them reach runtime code.
- Set beforeunload returnValue so the unsaved-changes prompt shows in
  all browsers; reject non-finite numbers in settings validation.
- Fix README/CN/JA docs that claimed the panel auto-generates
  AI_MODELS_CONFIG (providers are merged at read time, not written).
- Add unit tests for corrupted-file value filtering and provider
  schema validation.

* docs: move admin panel details to dedicated docs/{en,cn,ja}/admin-panel.md

The READMEs now carry a short blurb + link, matching the existing
per-topic docs (docker.md, ai-providers.md, ...). Removes the ~22-line
inline section and the duplicated data/settings.json mentions.

* fix(admin): address follow-up Copilot findings on the prior fixes

- loadAdminProviders now validates against a stored-shape schema where
  secrets are plain strings, so a hand-edited ADMIN_PROVIDERS holding an
  {isSet} marker is dropped instead of later crashing maskSecret().
- loadSettings guards against array values (typeof [] === 'object'),
  which would otherwise overlay numeric keys onto process.env.
- Admin SecretInput uses the bare id so the shared component's
  <Label htmlFor> stays associated (only one ProviderDetail mounts).
- Add tests: marker-secret rejection, array-values guard, bedrock
  multi-secret round-trip.

lib/i18n/dictionaries/zh-Hant.json

@@ -402,6 +402,152 @@
         "showUnvalidatedModels": "顯示未驗證的模型",
         "allModelsShown": "顯示所有模型（包括未驗證的）",
         "unvalidatedModelWarning": "此模型尚未驗證",
-        "serverDefaultModel": "伺服器預設模型"
+        "serverDefaultModel": "伺服器預設模型",
+        "showValue": "顯示值",
+        "hideValue": "隱藏值"
+    },
+    "admin": {
+        "title": "管理員設定",
+        "loginPrompt": "輸入管理員密碼（即 ADMIN_PASSWORD 環境變數）以管理伺服器設定。",
+        "password": "密碼",
+        "signIn": "登入",
+        "signingIn": "正在登入…",
+        "loginFailed": "登入失敗",
+        "precedence": "檔案覆蓋環境變數 · 環境變數覆蓋預設值",
+        "notWritable": "此部署環境下設定檔不可寫入（無伺服器平台沒有持久化磁碟）。設定以唯讀方式顯示——請改用環境變數進行設定。",
+        "settingGroups": "設定分組",
+        "enabled": "已啟用",
+        "disabled": "已停用",
+        "enableGroup": "啟用 {group}",
+        "unsavedChanges": "有未儲存的變更",
+        "saved": "設定已儲存，變更立即生效。",
+        "saveFailed": "儲存失敗。請檢查網路連線後重試。",
+        "invalidSettings": "部分設定無效。",
+        "discard": "捨棄",
+        "saveChanges": "儲存變更",
+        "saving": "正在儲存…",
+        "sourceSaved": "已儲存",
+        "sourceEnv": "環境變數",
+        "sourceSavedTitle": "在管理員設定檔中設定",
+        "sourceEnvTitle": "透過環境變數設定",
+        "restartRequired": "需要重新啟動",
+        "modified": "已修改",
+        "notSet": "未設定",
+        "savedReplace": "已儲存（{hint}）——輸入以取代",
+        "showValue": "顯示值",
+        "hideValue": "隱藏值",
+        "removeValue": "移除值",
+        "removeValueTitle": "移除已儲存的值",
+        "resetToDefault": "重設為預設",
+        "models": "模型",
+        "modelsDescription": "面向所有使用者的伺服器端 provider 與模型——無需個人 API 金鑰。當使用者未選擇模型時，使用預設 provider 的第一個模型。",
+        "addProviderHint": "新增一個 provider，為所有使用者提供伺服器端模型。",
+        "selectProviderHint": "選擇或新增一個 provider 以設定其憑證和模型。",
+        "addProviderToOfferModels": "至少新增一個模型，才能向使用者開放此 provider。",
+        "managedViaEnv": "（透過環境變數管理）",
+        "envReadOnly": "在 AI_MODELS_CONFIG / ai-models.json 中定義——此處唯讀。請編輯環境設定以變更。",
+        "defaultModel": "預設模型",
+        "noModelsConfigured": "未設定模型",
+        "modelCount": "{count} 個模型",
+        "modelCountPlural": "{count} 個模型",
+        "default": "預設",
+        "setAsDefault": "設為預設 provider",
+        "defaultProvider": "預設 provider",
+        "modelIdPlaceholder": "模型 ID…",
+        "addModel": "新增模型",
+        "suggested": "推薦",
+        "test": "測試",
+        "testOk": "正常（{ms} 毫秒）",
+        "testFailed": "失敗",
+        "removeModel": "移除 {model}",
+        "deleteProviderTitle": "刪除 {name}？",
+        "deleteProviderDesc": "儲存後，其憑證和模型將從伺服器上移除。",
+        "cancel": "取消",
+        "delete": "刪除",
+        "groups": {
+            "generation": {
+                "title": "生成",
+                "description": "套用於所有聊天請求的輸出參數。"
+            },
+            "access": {
+                "title": "存取控制",
+                "description": "限制誰可以使用此部署。"
+            },
+            "features": {
+                "title": "功能",
+                "description": "選用功能和安全開關。"
+            },
+            "observability": {
+                "title": "可觀測性",
+                "description": "對 LLM 呼叫進行 Langfuse 追蹤。"
+            },
+            "quota": {
+                "title": "配額與速率限制",
+                "description": "按 IP 的用量限制。強制執行需要 DynamoDB 表。"
+            }
+        },
+        "settings": {
+            "TEMPERATURE": {
+                "label": "溫度",
+                "description": "對於拒絕溫度參數的推理模型，請留空。"
+            },
+            "MAX_OUTPUT_TOKENS": {
+                "label": "最大輸出 token 數"
+            },
+            "ACCESS_CODE_LIST": {
+                "label": "存取碼",
+                "description": "以逗號分隔的清單。使用者需輸入其中之一才能聊天。留空 = 開放存取。"
+            },
+            "ENABLE_VLM_VALIDATION": {
+                "label": "VLM 圖表驗證",
+                "description": "使用視覺模型對產生的圖表進行視覺化驗證。"
+            },
+            "VALIDATION_MODEL": {
+                "label": "驗證模型",
+                "description": "留空時回退到預設 AI 模型。"
+            },
+            "VALIDATION_TIMEOUT": {
+                "label": "驗證逾時（毫秒）"
+            },
+            "ENABLE_HISTORY_XML_REPLACE": {
+                "label": "歷史 XML 壓縮",
+                "description": "用占位符取代歷史記錄中的舊圖表 XML。"
+            },
+            "ALLOW_PRIVATE_URLS": {
+                "label": "允許私有 URL",
+                "description": "關閉以阻擋對私有 IP 和內部主機名的請求（SSRF 防護）。"
+            },
+            "LANGFUSE_PUBLIC_KEY": {
+                "label": "Langfuse Public Key"
+            },
+            "LANGFUSE_SECRET_KEY": {
+                "label": "Langfuse Secret Key"
+            },
+            "LANGFUSE_BASEURL": {
+                "label": "Langfuse Base URL"
+            },
+            "DAILY_REQUEST_LIMIT": {
+                "label": "每日請求上限",
+                "description": "每個 IP 每天。"
+            },
+            "DAILY_TOKEN_LIMIT": {
+                "label": "每日 token 上限",
+                "description": "每個 IP 每天。"
+            },
+            "TPM_LIMIT": {
+                "label": "每分鐘 token 數"
+            },
+            "DYNAMODB_QUOTA_TABLE": {
+                "label": "DynamoDB 表",
+                "description": "留空時配額強制執行被停用。"
+            },
+            "DYNAMODB_REGION": {
+                "label": "DynamoDB 區域"
+            },
+            "QUOTA_TIMEZONE": {
+                "label": "配額時區",
+                "description": "每日重置邊界所用的時區。"
+            }
+        }
     }
 }