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

env.example

@@ -8,6 +8,7 @@ AI_PROVIDER=bedrock
 # Tip: For a single-provider quick multi-model setup, list comma-separated model IDs.
 # The first one becomes the default and the rest appear in the model picker.
 # For multiple providers or custom apiKeyEnv/baseUrlEnv, use AI_MODELS_CONFIG / ai-models.json instead.
+# There, apiKeyEnv may also be an array of env var names: each request picks one that has a value at random.
 # Example: AI_MODEL=doubao-seed-1-8-251215,doubao-seed-1-6-flash,doubao-seed-1-6-pro
 AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 
@@ -28,8 +29,6 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # OpenAI Configuration
 # OPENAI_API_KEY=sk-...
 # OPENAI_BASE_URL=https://api.openai.com/v1  # Optional: Custom OpenAI-compatible endpoint
-# OPENAI_ORGANIZATION=org-...  # Optional
-# OPENAI_PROJECT=proj_...      # Optional
 # Note: o1/o3/gpt-5 models automatically enable reasoning summary (default: detailed)
 # OPENAI_REASONING_EFFORT=low   # Optional: Reasoning effort (minimal/low/medium/high) - for o1/o3/gpt-5
 # OPENAI_REASONING_SUMMARY=detailed  # Optional: Override reasoning summary (none/brief/detailed)
@@ -142,6 +141,18 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # ADMIN_PASSWORD=your-admin-password
 # SETTINGS_FILE=./data/settings.json  # Optional: custom settings file location
 
+# Quota & Validation (Optional, also editable in the admin panel)
+# Quota enforcement is disabled unless DYNAMODB_QUOTA_TABLE is set.
+# DYNAMODB_QUOTA_TABLE=next-ai-draw-io-quota  # DynamoDB table that stores the per-IP counters
+# DYNAMODB_REGION=ap-northeast-1              # Region of the table
+# QUOTA_TIMEZONE=UTC                          # Timezone for the daily reset boundary
+# DAILY_REQUEST_LIMIT=10                      # Requests per IP per day
+# DAILY_TOKEN_LIMIT=200000                    # Tokens per IP per day
+# TPM_LIMIT=20000                             # Tokens per minute
+# ENABLE_VLM_VALIDATION=true                  # Visually validate generated diagrams with a vision model
+# VALIDATION_MODEL=                           # Falls back to the default AI model when empty
+# VALIDATION_TIMEOUT=30000                    # Validation timeout in ms
+
 # Draw.io Configuration (Optional)
 # By default the app serves its own copy of draw.io from /drawio (downloaded into
 # public/drawio by `npm run dev` / `npm run build`). Same origin is required for
@@ -156,9 +167,7 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # NEXT_PUBLIC_BASE_PATH=/nextaidrawio
 
 # PDF Input Feature (Optional)
-# Enable PDF file upload to extract text and generate diagrams
-# Enabled by default. Set to "false" to disable.
-# ENABLE_PDF_INPUT=true
+# PDF file upload to extract text and generate diagrams
 # NEXT_PUBLIC_MAX_EXTRACTED_CHARS=150000  # Max characters for PDF/text extraction (default: 150000)
 
 # Security Settings (Optional)
@@ -184,6 +193,10 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # It also turns off the MCP server advertisement in Settings > About.
 # NEXT_PUBLIC_SELFHOSTED=true
 
+# Analytics (Optional)
+# Google Analytics measurement ID; nothing is loaded when unset
+# NEXT_PUBLIC_GA_ID=G-XXXXXXXXXX
+
 # Minimax Configuration (Optional)
 # Get your API key from: https://platform.minimaxi.com/docs/guides/models-intro
 # MINIMAX_API_KEY=your_minimax_api_key