perf: Migrate all-files search to Web Worker for UI responsiveness

[pedropaulovc]

Problem

The current "Find in All Files" (Ctrl+Shift+F) search executes synchronously on the main thread. For large PRs with many files, this could block the UI and make the application feel unresponsive.

Location: src/app/[owner]/[repo]/[number]/page.tsx - handleExecuteSearch callback

Proposed Solution

Migrate the search logic to a Web Worker to keep the main thread free for UI updates.

Implementation Approach

1. Create a new search worker in src/features/search/workers/search-worker.ts
2. Move the search loop logic (file filtering, patch parsing, match finding) to the worker
3. Use postMessage to send search parameters and receive results
4. Show a progress indicator during search (already have isSearching state)
5. Support cancellation for long-running searches

Benefits

• UI remains responsive during search
• Can show real-time progress (X of Y files searched)
• Can cancel in-flight searches when user types new query
• Consistent with existing src/features/diff/workers/diff-engine.ts pattern

Acceptance Criteria

• Search executes in Web Worker, not main thread
• UI remains responsive during search of large PRs (100+ files)
• Progress indicator shows search status
• User can cancel in-progress search
• Unit tests for worker message handling

Context

Identified during PR #292 code review by Copilot.