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

app/api/log-feedback/route.ts

@@ -1,6 +1,5 @@
-import { randomUUID } from "crypto"
 import { z } from "zod"
-import { getLangfuseClient } from "@/lib/langfuse"
+import { getLangfuseClient, scoreLatestTrace } from "@/lib/langfuse"
 import { getUserIdFromRequest } from "@/lib/user-id"
 
 const feedbackSchema = z.object({
@@ -37,75 +36,24 @@ export async function POST(req: Request) {
     const userId = getUserIdFromRequest(req)
 
     try {
-        // Find the most recent chat trace for this session to attach the score to
-        const tracesResponse = await langfuse.api.trace.list({
+        // Score the session's latest chat trace; without one, a standalone
+        // feedback trace carries the score
+        await scoreLatestTrace(langfuse, {
             sessionId,
-            limit: 1,
+            name: "user-feedback",
+            value: feedback === "good" ? 1 : 0,
+            comment: `User gave ${feedback} feedback`,
+            fallbackTrace: {
+                name: "user-feedback",
+                userId,
+                input: { messageId, feedback },
+                metadata: {
+                    source: "feedback-button",
+                    note: "standalone - no chat trace found",
+                },
+            },
         })
 
-        const traces = tracesResponse.data || []
-        const latestTrace = traces[0]
-
-        if (!latestTrace) {
-            // No trace found for this session - create a standalone feedback trace
-            const traceId = randomUUID()
-            const timestamp = new Date().toISOString()
-
-            await langfuse.api.ingestion.batch({
-                batch: [
-                    {
-                        type: "trace-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: traceId,
-                            name: "user-feedback",
-                            sessionId,
-                            userId,
-                            input: { messageId, feedback },
-                            metadata: {
-                                source: "feedback-button",
-                                note: "standalone - no chat trace found",
-                            },
-                            timestamp,
-                        },
-                    },
-                    {
-                        type: "score-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: randomUUID(),
-                            traceId,
-                            name: "user-feedback",
-                            value: feedback === "good" ? 1 : 0,
-                            comment: `User gave ${feedback} feedback`,
-                        },
-                    },
-                ],
-            })
-        } else {
-            // Attach score to the existing chat trace
-            const timestamp = new Date().toISOString()
-
-            await langfuse.api.ingestion.batch({
-                batch: [
-                    {
-                        type: "score-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: randomUUID(),
-                            traceId: latestTrace.id,
-                            name: "user-feedback",
-                            value: feedback === "good" ? 1 : 0,
-                            comment: `User gave ${feedback} feedback`,
-                        },
-                    },
-                ],
-            })
-        }
-
         return Response.json({ success: true, logged: true })
     } catch (error) {
         console.error("Langfuse feedback error:", error)