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

tests/unit/edgeone-function.test.ts

@@ -1,22 +1,37 @@
 // @vitest-environment node
-import { describe, expect, it } from "vitest"
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest"
 import { onRequest } from "@/edge-functions/api/edgeai/chat/completions"
 
+// EdgeOne's global AI object, answering with an empty stream
+beforeEach(() => {
+    vi.stubGlobal("AI", {
+        chatCompletions: async () => new ReadableStream(),
+    })
+})
+afterEach(() => {
+    vi.unstubAllGlobals()
+})
+
 function request(headers: Record<string, string>): Request {
     return new Request("http://localhost/api/edgeai/chat/completions", {
         method: "POST",
         headers,
-        // Non-streaming requests return a mock reply without calling AI
         body: JSON.stringify({ messages: [{ role: "user", content: "hi" }] }),
     })
 }
 
 const json = { "Content-Type": "application/json" }
 
+// An accepted request streams the model's reply
+function expectStream(res: Response) {
+    expect(res.status).toBe(200)
+    expect(res.headers.get("content-type")).toContain("text/event-stream")
+}
+
 describe("EdgeOne chat completions function", () => {
     it("sends no CORS headers", async () => {
         const res = await onRequest({ request: request(json), env: {} })
-        expect(res.status).toBe(200)
+        expectStream(res)
         expect(res.headers.get("access-control-allow-origin")).toBeNull()
     })
 
@@ -40,7 +55,7 @@ describe("EdgeOne chat completions function", () => {
             }),
             env: {},
         })
-        expect(withCharset.status).toBe(200)
+        expectStream(withCharset)
     })
 
     it("checks the access code when ACCESS_CODE_LIST is set", async () => {
@@ -52,11 +67,11 @@ describe("EdgeOne chat completions function", () => {
             request: request({ ...json, "x-access-code": "secret" }),
             env,
         })
-        expect(ok.status).toBe(200)
+        expectStream(ok)
     })
 
     it("lets requests through when env is unavailable", async () => {
         const res = await onRequest({ request: request(json) })
-        expect(res.status).toBe(200)
+        expectStream(res)
     })
 })