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

components/dev-xml-simulator.tsx

@@ -1,7 +1,6 @@
 "use client"
 
 import { useEffect, useRef, useState } from "react"
-import { useDictionary } from "@/hooks/use-dictionary"
 import { prepareNewDiagram } from "@/packages/mcp-server/src/new-diagram.ts"
 
 // Dev XML presets for streaming simulator
@@ -135,15 +134,12 @@ const DEV_XML_PRESETS: Record<string, string> = {
 interface DevXmlSimulatorProps {
     setMessages: React.Dispatch<React.SetStateAction<any[]>>
     onDisplayChart: (xml: string) => void
-    onShowQuotaToast?: () => void
 }
 
 export function DevXmlSimulator({
     setMessages,
     onDisplayChart,
-    onShowQuotaToast,
 }: DevXmlSimulatorProps) {
-    const dict = useDictionary()
     const [devXml, setDevXml] = useState("")
     const [isSimulating, setIsSimulating] = useState(false)
     const [devIntervalMs, setDevIntervalMs] = useState(1)
@@ -180,7 +176,7 @@ export function DevXmlSimulator({
             parts: [
                 {
                     type: "text" as const,
-                    text: dict.dev.simulatingMessage,
+                    text: "[Dev] Simulating XML streaming",
                 },
             ],
         }
@@ -230,7 +226,7 @@ export function DevXmlSimulator({
             const lastMsg = updated[updated.length - 1] as any
             if (lastMsg?.role === "assistant" && lastMsg.parts?.[0]) {
                 lastMsg.parts[0].state = "output-available"
-                lastMsg.parts[0].output = dict.dev.successMessage
+                lastMsg.parts[0].output = "Successfully displayed the diagram."
                 lastMsg.parts[0].input = { xml }
             }
             return updated
@@ -247,12 +243,12 @@ export function DevXmlSimulator({
         <div className="border-t border-dashed border-orange-500/50 px-4 py-2 bg-orange-50/50 dark:bg-orange-950/30">
             <details>
                 <summary className="text-xs text-orange-600 dark:text-orange-400 cursor-pointer font-medium">
-                    {dict.dev.title}
+                    Dev: XML Streaming Simulator
                 </summary>
                 <div className="mt-2 space-y-2">
                     <div className="flex items-center gap-2">
                         <label className="text-xs text-muted-foreground whitespace-nowrap">
-                            {dict.dev.preset}
+                            Preset:
                         </label>
                         <select
                             onChange={(e) => {
@@ -264,7 +260,7 @@ export function DevXmlSimulator({
                             defaultValue=""
                         >
                             <option value="" disabled>
-                                {dict.dev.selectPreset}
+                                Select a preset...
                             </option>
                             {Object.keys(DEV_XML_PRESETS).map((name) => (
                                 <option key={name} value={name}>
@@ -277,19 +273,19 @@ export function DevXmlSimulator({
                             onClick={() => setDevXml("")}
                             className="px-2 py-1 text-xs text-muted-foreground hover:text-foreground border rounded"
                         >
-                            {dict.dev.clear}
+                            Clear
                         </button>
                     </div>
                     <textarea
                         value={devXml}
                         onChange={(e) => setDevXml(e.target.value)}
-                        placeholder={dict.dev.placeholder}
+                        placeholder="Paste mxCell XML here or select a preset..."
                         className="w-full h-24 text-xs font-mono p-2 border rounded bg-background"
                     />
                     <div className="flex items-center gap-4">
                         <div className="flex items-center gap-2 flex-1">
                             <label className="text-xs text-muted-foreground whitespace-nowrap">
-                                {dict.dev.interval}
+                                Interval:
                             </label>
                             <input
                                 type="range"
@@ -308,7 +304,7 @@ export function DevXmlSimulator({
                         </div>
                         <div className="flex items-center gap-2">
                             <label className="text-xs text-muted-foreground whitespace-nowrap">
-                                {dict.dev.chars}
+                                Chars:
                             </label>
                             <input
                                 type="number"
@@ -332,8 +328,8 @@ export function DevXmlSimulator({
                             className="px-3 py-1 text-xs bg-orange-500 text-white rounded hover:bg-orange-600 disabled:opacity-50 disabled:cursor-not-allowed"
                         >
                             {isSimulating
-                                ? dict.dev.streaming
-                                : `${dict.dev.simulate} (${devChunkSize} chars/${devIntervalMs}ms)`}
+                                ? "Streaming..."
+                                : `Simulate (${devChunkSize} chars/${devIntervalMs}ms)`}
                         </button>
                         {isSimulating && (
                             <button
@@ -343,16 +339,7 @@ export function DevXmlSimulator({
                                 }}
                                 className="px-3 py-1 text-xs bg-red-500 text-white rounded hover:bg-red-600"
                             >
-                                {dict.dev.stop}
-                            </button>
-                        )}
-                        {onShowQuotaToast && (
-                            <button
-                                type="button"
-                                onClick={onShowQuotaToast}
-                                className="px-3 py-1 text-xs bg-purple-500 text-white rounded hover:bg-purple-600"
-                            >
-                                {dict.dev.testQuotaToast}
+                                Stop
                             </button>
                         )}
                     </div>