Comprehensive cleanup and modernization across UI, tests, and CI

- Split monolithic ChatView.browser.tsx into 12 modular test files
- Moved first-turn thread titles to server-owned provider-native generation
- Changed diff.toggle keybinding and added capture-phase keyboard event handling
- Simplified DiffPanel by removing lazy loading
- Updated CI to use ubuntu-24.04 runner and added release validation gates
- Added GitHub-hosted desktop installers and release build automation
- Updated documentation structure and added discovered agents support
- Fixed GitManager missing-directory handling and stale PR fixtures
- Added missing GitManager invalidateStatus mock to server tests
- Show BigCode logo centered on empty state instead of text

Author: youpele52