feat(web): add keyword search to docs hub and FAQ pages

[idling11]

Add client-side keyword search to two of the most content-heavy pages on codewhale.net:

Docs hub (/docs):

• New DocsSearch client component with real-time filtering across all doc topics (label, description, source files, category — in both EN and ZH)
• Search input with / keyboard shortcut to focus, clear button, result count, and no-results state
• Matched text is highlighted inline
• The topic grid and category sections update live as you type

FAQ page (/faq):

• New FaqSearch client component wrapping the existing FAQ item list
• Full-text matching across question text, answer content (extracted from React nodes), and source citations
• Same UX patterns: / shortcut, highlight, result count

Shared logic:

• web/lib/search-utils.ts extracts the pure filtering functions for unit testability
• web/lib/search-utils.test.ts: 19 tests covering haystack building, locale-cross-matching, case-insensitivity, and edge cases

Styling:

• .search-input and mark.search-highlight classes in globals.css, using existing CSS variables for consistency with the site theme

Summary

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features
• cargo test --workspace --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes
• Harvested/co-authored credit uses a GitHub numeric noreply address