refactor: remove dead code and over-engineering found by two audits (#986)

## Problem

Two audits of the repository (a workflow of finder, dedup and per-file verifier agents, run on 2026-10-11 against 1f4141e and again against 07f3e3c after a first round of refactoring) found dead code and features that were more machinery than the app needs: an Electron configuration-preset system with its own settings window, three copies of the about page, a hand-written `.env` parser, an "extended" system prompt nobody selected, two data migrations for formats from 2025-12 and 2026-01, a server-side example cache, type-to-confirm provider deletion, 81 unused dictionary keys, wrapper components around single call sites, and a long tail of unused exports, props, options and tests.

## Change

Every item below was either approved one by one or covered by "fix all of the others".

- **Electron:** the configuration-preset system is gone (settings window, preload, IPC, menu entry, dev watcher); `config-manager.ts` keeps only `userLocale`; `.env` files are parsed with `dotenv`. Entitlements file and a duplicate CI workflow removed; `test.yml` gets a type check and concurrency.
- **Server:** no extended prompt, no example cache, no `ENABLE_HISTORY_XML_REPLACE`, no message metadata or `hasConfig`; the connection test is one shared helper; EdgeOne loses its mock branch and model aliases; `DEBUG_LLM_PAYLOAD` logs message structure instead of payloads; Langfuse scoring helper; `apiKeyEnv` arrays kept and documented in `env.example` and the provider docs.
- **Pages:** one about page per language from a single component (old `/cn` and `/ja` URLs redirect with 308); layout uses `hasLocale`; the GitHub icon is a local component instead of `react-icons`; Cloudflare deploys without an R2 cache by default.
- **Chat and canvas:** the legacy `diagramHistory` conversion, the localStorage chat migration and the templates storage are gone; `chat-engine` reads the diagram through the context's refs; the diagram context has one tagged export path (`requestTaggedExport` / `fetchDiagramXml`); the screenshot check uses fetch + JSON and `generateText` with `Output.object`; `load-diagram` accepts `<svg:svg>` and strips the XML prolog.
- **Settings UI:** `model-selector` inlines its Dialog, Command and Tooltip; `ai-elements/model-selector`, `button-with-tooltip`, `error-toast` removed; no type-to-confirm, no footer, no `bedrockApiKey`; `use-model-config` loses the old-format migration; one reasoning component; the shimmer is CSS (no `motion`, no `@radix-ui/react-use-controllable-state`); 81 dictionary keys removed in all four languages.
- **MCP server:** `registerTool` / `ok` / `fail` replace per-tool try/catch; `syncFromBrowser`, `commitDocument`, `describePages`; the hand-written XML syntax check, unused result fields, `pickPageSelector`, `expandStyles` and the `host` attribute handling are gone; tests get a shared `setup.ts` and drive the server through the SDK client.
- **Prompt:** the web app's style rules come from the MCP drawing guide (`styleRules`), byte-identical to before.
- **Tests:** shared e2e helpers (`openApp`, `sse`, `toolCallEvents`, `chunkedReplies`); placeholder specs dropped; `settings-review-fixes` split by topic; unit tests use `vi.stubEnv`.
- **Dependencies removed:** `@xmldom/xmldom`, `remark-gfm`, `@anthropic-ai/tokenizer`, `@testing-library/user-event`, `concurrently`, `wait-on`, `eslint`, `eslint-config-next`, `@vitest/coverage-v8`, `shx`, `motion`, `@radix-ui/react-use-controllable-state`, `react-icons`.

Source lines: 74,983 before, 67,418 after (excluding lockfiles).

The last commit merges #984 (the canvas shell). Where the shell, the classic page or their tests depend on a wire format or signature this branch had changed, main's version wins (export fields of the poll response, `addHistory` signature); where main only extended code this branch restructured, main's change is carried into the new structure (`start_session`, `get_selection`, `prepareNewDiagram`, the diagram context's `requestExport`).

## Testing

- `tsc --noEmit` for the web app, `packages/mcp-server`, its shell and `electron`: clean
- `biome ci`: 0 errors (37 pre-existing warnings)
- Root `vitest run`: 719 passed; `packages/mcp-server` `vitest run`: 372 passed
- `next build`: clean
- Playwright, whole web suite on the dev server (4 workers): 280 tests, 279 passed; `language.spec.ts` "a stale offer to switch without saving does nothing once a message is being sent" failed under parallel load and passed alone, as it does on main

lib/admin/providers.ts

@@ -115,7 +115,7 @@ export function loadAdminProviders(): StoredAdminProvider[] {
     }
 }
 
-export type MaskedAdminProvider = Omit<
+type MaskedAdminProvider = Omit<
     StoredAdminProvider,
     (typeof SECRET_FIELDS)[number]
 > & {
@@ -278,15 +278,9 @@ export function deriveEnvUpdates(
     return updates
 }
 
-// Every settings key the panel may have written for a given list.
-// AI_MODELS_CONFIG is included to clean up values written by older
-// versions of the panel (it is no longer written).
+// Every settings key the panel may have written for a given list
 function derivedEnvKeys(list: StoredAdminProvider[]): string[] {
-    const keys = new Set<string>([
-        "AI_MODELS_CONFIG",
-        "AI_PROVIDER",
-        "AI_MODEL",
-    ])
+    const keys = new Set<string>(["AI_PROVIDER", "AI_MODEL"])
     const indexByProvider = new Map<ProviderName, number>()
     for (const p of list) {
         const index = indexByProvider.get(p.provider) ?? 0

lib/admin/settings-registry.ts

@@ -10,7 +10,7 @@
 // - ADMIN_PASSWORD / SETTINGS_FILE: bootstrap values, env-only to avoid lockout
 // - Per-provider reasoning/thinking tuning vars: env-only (see env.example)
 
-export type SettingType = "string" | "secret" | "number" | "boolean" | "enum"
+type SettingType = "string" | "secret" | "number" | "boolean"
 
 export interface SettingDef {
     key: string
@@ -18,7 +18,6 @@ export interface SettingDef {
     type: SettingType
     label: string
     description?: string
-    options?: string[]
     min?: number
     max?: number
     placeholder?: string
@@ -123,13 +122,6 @@ export const SETTINGS_REGISTRY: SettingDef[] = [
         label: "Validation Timeout (ms)",
         min: 1000,
     },
-    {
-        key: "ENABLE_HISTORY_XML_REPLACE",
-        group: "features",
-        type: "boolean",
-        label: "History XML Compression",
-        description: "Replace old diagram XML in history with placeholders.",
-    },
     {
         key: "ALLOW_PRIVATE_URLS",
         group: "features",

lib/admin/settings.ts

@@ -32,7 +32,7 @@ globalState.__adminSettingsState ??= {
 }
 const state = globalState.__adminSettingsState
 
-export function getSettingsPath(): string {
+function getSettingsPath(): string {
     const custom = process.env.SETTINGS_FILE
     if (custom && custom.trim().length > 0) return custom
     return path.join(process.cwd(), "data", "settings.json")