feat: v3.0.9 — update checker, custom paths, session pagination

- Add GitHub release update checker with auto-check on startup
- Sidebar shows amber dot badge when update available
- About page: check button, update card with release notes + download link
- Custom platform paths in settings with native folder picker dialog
- Session list pagination (50 per page + Load More button)
- BufReader optimization for Codex scan_summary
- Move runtime info section to About page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: voidcraft-dev