feat(search): Add GitHub Code Search support

[vredchenko]

Summary

Add GitHub Code Search to search repository file contents. This adds a "Code" filter chip to the SearchPalette.

Technical Details

• API endpoint: GET /search/code?q={query}+repo:owner/repo
• Rate limiting: 30 requests per minute (authenticated users)
• Constraints: Only searches default branch, files smaller than 384 KB

Implementation

1. Add searchGitHubCode() function in useSearch.ts
2. Add "Code" to SearchSourceType
3. Add "Code" filter chip to SearchPalette
4. Configure in webui-config.ts

Priority

Future enhancement - not required for MVP.

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From PR #124 review comment (Item 7)