feat(search): Add regex search support for documentation

[vredchenko]

Summary

Add optional regex search capabilities for power users searching documentation.

Challenge

MiniSearch doesn't natively support regex - uses fuzzy/prefix matching instead.

Potential Approaches

1. Post-filter: Run MiniSearch first, then filter with RegExp.test()
2. Separate path: Detect regex patterns (e.g., /pattern/), bypass MiniSearch, filter indexed docs directly
3. UI options: Toggle switch, prefix detection, or docs-only limitation

Files to Modify

• webui/src/hooks/useSearch.ts
• webui/src/components/widgets/SearchPalette/SearchPalette.tsx

Priority

Future enhancement - not required for MVP. Medium complexity due to performance considerations.

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From PR #124 review comment (Item 8)