fix(account): 优化账号菜单和表单提示

[xyh202131]

Summary

• 账号菜单支持点击外部、Escape、失焦和指针离开关闭
• 账号侧栏在桌面端保持稳定定位
• 优化昵称、验证码和密码输入提示，并统一默认头像

Verification

• npm test -- --run src/app/layout/app-header.test.tsx src/pages/account/index.test.tsx（55 passed）
• npm run typecheck
• npm run lint
• npm run build

Refs #783

[vercel]

Deployment failed for project windup with the following error:

Resource is limited - try again in 24 hours (more than 100, code: "api-deployments-free-per-day").

Learn More: https://vercel.com/huyan-s-projects1?upgradeToPro=build-rate-limit

[github-actions]

⚠️ @xyh202131，此 PR 尚未关联 issue。请在 PR 描述中使用 Closes #123 等关闭关键字；更新描述后，此提醒将自动标记为已解决。

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
frontend/src/app/layout/app-header.tsx	71.42%	2 Missing and 2 partials ⚠️
frontend/src/pages/account/index.tsx	0.00%	0 Missing and 2 partials ⚠️

@@            Coverage Diff             @@
##             main     #913      +/-   ##
==========================================
- Coverage   86.50%   86.21%   -0.30%     
==========================================
  Files         304      305       +1     
  Lines       23830    23853      +23     
  Branches     3597     3605       +8     
==========================================
- Hits        20615    20565      -50     
- Misses       2249     2325      +76     
+ Partials      966      963       -3     

Flag	Coverage Δ
backend	91.26% <ø> (ø)
frontend	79.99% <62.50%> (-0.65%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
frontend/src/pages/account/index.tsx	79.38% <0.00%> (-0.31%)	⬇️
frontend/src/app/layout/app-header.tsx	91.74% <71.42%> (-3.05%)	⬇️

... and 5 files with indirect coverage changes

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[fennoai]

I reviewed the complete fixed-range diff for the account menu and account settings changes. The account form guidance and sticky desktop navigation are consistent with their consumers, but the new pointer-dismiss interaction introduces a mouse usability regression for the menu items. Local focused tests and typecheck were not runnable because this checkout has no executable vitest/tsc dependencies.