feat: PySide6 migration, theme engine, GUI virtualization, tag system

- Migrate all PyQt6 imports to PySide6 (LGPL compliance)
- Add ThemeEngine with 3 switchable themes (Light, Dark, High Contrast)
- Central QSS stylesheet system (350+ lines), no more scattered styles
- Migrate FavoritesView and GlobalSearchView to QListView + custom delegate
- Add TagManager with full CRUD, DB persistence, AND-filter logic
- Tag chips in item delegates, context menu tag assignment
- 20 new unit tests for tag system (99/99 total passing)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>