feat(electron): add Playwright-driven e2e tests for the demo app

[Harriet-star-glitch]

Summary / 摘要

Adds a real end-to-end test layer (Roadmap #25) that drives the packaged electron-demo build via Playwright's _electron — real Chromium, real main process, real IPC — instead of jsdom.

Motivation / 背景与动机

• Issue:
• Roadmap (docs/ROADMAP.md): 完成笔试题 #25 End-to-end testing, P1, marked done in this PR. Row says "Needs OpenSpec: No", so no proposal opened.
• OpenSpec change: none (test infra, exempted per CONTRIBUTING.md §3.1)

Changes / 变更内容

• apps/electron-demo:
  • e2e/driver.ts: launchApp()/closeApp() helper. Resolves the electron binary via the electron package's own export (portable, no hardcoded paths) and strips ELECTRON_RUN_AS_NODE from the spawned env — some sandboxed/CI shells set it to stop a stray electron invocation from opening a GUI window, which would otherwise make the launch handshake silently time out.
  • e2e/smoke.e2e.test.ts: three golden-path scenarios — app launches to an empty editor, live-preview renders typed markdown (heading/bold/italic), and a GFM task checkbox toggles via a real DOM click.
  • vitest.e2e.config.ts + new test:e2e script: separate from the default pnpm test on purpose (needs a prior pnpm build + a display).
  • tsconfig.json: include e2e/**/*.ts so it's typechecked.
  • package.json: new playwright-core devDependency.
• vitest.config.ts (root): excludes **/e2e/**. Without this the root pnpm test picked the new file up anyway (vitest's default include glob matches any *.test.ts regardless of the .e2e. infix) and ran it under jsdom with no built app — confirmed by actually running pnpm test before adding the exclude and watching it try (and fail the isolation goal, though the individual test happened to still pass).
• docs/ROADMAP.md: 完成笔试题 #25 → done, documents how to run it and what's out of scope for this first pass.

Explicitly out of scope (flagging so it doesn't look like an oversight):

• CI wiring (Roadmap feat(search): 增加整词匹配支持 #26, "missing e2e gate") — different roadmap item, kept separate per CONTRIBUTING.md "one PR, one concern".
• Vault/file-IO e2e coverage — the vault flow goes through a real native OS file picker dialog (dialog.showOpenDialog), which Playwright can't drive directly; bypassing it needs its own design (e.g. a test-only IPC shortcut) rather than folding it into this PR.

Testing / 测试

• pnpm test passes (515/515) — confirmed the e2e file does NOT run here after the vitest.config.ts exclude fix
• pnpm typecheck passes repo-wide
• pnpm build passes repo-wide
• New e2e suite: pnpm --filter @floatboat/nexus-electron-demo build && pnpm --filter @floatboat/nexus-electron-demo test:e2e — 3/3 passing, run 4 times total (~3s each) with no flakiness observed
• Manual UI check in electron-demo — n/a, this PR is the automated version of that manual check

Compliance / 合规自检

• CLA signed — will sign if/when the bot prompts
• AI disclosure: this PR's functional code is primarily AI-generated, same as my prior PR (feat(live-preview): add grip-handle drag reorder for list items #132) on this repo. I asked Claude (Anthropic's Claude Code) to pick the hardest of the roadmap items I'd surveyed and implement it end-to-end — the driver, the tests, the config isolation fix, and the roadmap update. I reviewed the diff, the actual test:e2e output across multiple runs, and the repo-wide pnpm test/typecheck/build results, and caught (and had it fix) one real bug along the way: the checkbox e2e assertion originally trusted a stale DOM reference after .click(), which the app's own decoration-keying behavior detaches. I have not independently rewritten the implementation myself. Disclosing this explicitly per GOVERNANCE.md §6.2 rather than checking "not AI-generated" — understand maintainers may decide this doesn't meet that bar.
• New dependencies: playwright-core@^1.61.1 (devDependency, apps/electron-demo only) — Apache-2.0 license — needed to drive the packaged Electron app from outside its process; -core variant specifically because it doesn't bundle/download its own browser binaries, it drives the project's own already-installed Electron binary.
• No build artifacts committed
• No secrets committed

Checklist / 自检清单

• Title follows Conventional Commits
• Public API changes update package README — n/a, electron-demo is a private app, not a published package
• Touched live-preview-table.ts — n/a, not touched in this PR
• New capability / breaking change → OpenSpec — test infra is exempt per CONTRIBUTING.md §3.1 ("bug fixes, internal refactors, dependency bumps, test/doc additions" don't need a proposal); confirmed against the ROADMAP.md row's own "Needs OpenSpec: No"
• Change aligns with project scope

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