feat(Timer): add new Timer component

[zehuichan]

Summary

• Add Timer component for real-time elapsed (count-up) display, mirroring CountDown API and docs patterns
• Add @vant/use useTimer composable with optional maxTime support
• Cover unit tests, bilingual docs, demo, theme vars, and site nav registration

Test plan

• pnpm --dir ./packages/vant-use build
• pnpm exec rstest run src/timer/test/index.spec.tsx (in packages/vant)
• pnpm dev and verify Timer demo (basic / format / millisecond / max-time / custom style / manual control)
• Confirm docs pages: Timer (zh/en) and useTimer (zh/en) render in the site nav

Made with Cursor

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.44%. Comparing base (ec5b45b) to head (53d6736).
⚠️ Report is 282 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/vant/src/timer/Timer.tsx	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13883      +/-   ##
==========================================
- Coverage   89.60%   89.44%   -0.17%     
==========================================
  Files         257      260       +3     
  Lines        7013     6734     -279     
  Branches     1736     1684      -52     
==========================================
- Hits         6284     6023     -261     
+ Misses        384      376       -8     
+ Partials      345      335      -10     

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