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

docs/en/cloudflare-deploy.md

@@ -68,47 +68,15 @@ Fill in the same values there.
 
 ## Step 3 — Choose your deployment type
 
-### Option A: Deploy WITHOUT R2 (Simple, Free)
+### Option A: Deploy WITHOUT R2 (Default, Free)
 
-If you don't need ISR caching, you can deploy without R2:
-
-**1. Use simple `open-next.config.ts`:**
-
-```ts
-import { defineCloudflareConfig } from "@opennextjs/cloudflare/config"
-
-export default defineCloudflareConfig({})
-```
-
-**2. Use simple `wrangler.jsonc` (without r2_buckets):**
-
-```jsonc
-{
-  "$schema": "node_modules/wrangler/config-schema.json",
-  "main": ".open-next/worker.js",
-  "name": "next-ai-draw-io-worker",
-  "compatibility_date": "2025-12-08",
-  "compatibility_flags": ["nodejs_compat", "global_fetch_strictly_public"],
-  "assets": {
-    "directory": ".open-next/assets",
-    "binding": "ASSETS"
-  },
-  "services": [
-    {
-      "binding": "WORKER_SELF_REFERENCE",
-      "service": "next-ai-draw-io-worker"
-    }
-  ]
-}
-```
-
-Skip to **Step 4**.
+This is how the repository is configured: `open-next.config.ts` is `defineCloudflareConfig({})` and `wrangler.jsonc` has no `r2_buckets`. Nothing to change here. Skip to **Step 4**.
 
 ---
 
-### Option B: Deploy WITH R2 (Full ISR Support)
+### Option B: Deploy WITH R2 (Optional, ISR caching)
 
-R2 enables **Incremental Static Regeneration (ISR)** caching. Requires a payment method on your Cloudflare account.
+R2 gives OpenNext a place to store **Incremental Static Regeneration (ISR)** pages. This app currently has no ISR pages, so you only need this step if you add some. Requires a payment method on your Cloudflare account.
 
 **1. Create an R2 bucket** in the Cloudflare Dashboard:
 
@@ -127,7 +95,7 @@ export default defineCloudflareConfig({
 })
 ```
 
-**3. Configure `wrangler.jsonc` (with R2):**
+**3. Add `r2_buckets` to `wrangler.jsonc`:**
 
 ```jsonc
 {
@@ -157,6 +125,8 @@ export default defineCloudflareConfig({
 
 > **Important:** The `bucket_name` must exactly match the name you created in the Cloudflare dashboard.
 
+> **Note:** Time-based revalidation (`revalidate` on a page or fetch) also needs a `queue` in `open-next.config.ts`. See the OpenNext Cloudflare caching docs.
+
 ---
 
 ## Step 4 — Register a workers.dev subdomain (first-time only)
@@ -264,4 +234,4 @@ npm run preview
 | API Routes | Yes | Yes |
 | Setup Complexity | Simple | Moderate |
 
-Choose **without R2** for testing or simple apps. Choose **with R2** for production apps that need ISR caching.
+The repository defaults to **without R2**. Choose **with R2** only if you add pages that need ISR caching.