refactor: centralize button styles in app.css
- Add global .btn, .btn-primary, .btn-secondary, .btn-danger, .btn-ghost classes
- Add --danger color CSS variables for multi-theme support
- Add .btn-sm and .btn-lg size modifiers
- Refactor all modals and views to use global button classes
- Remove 260+ lines of duplicated button CSS across components
Files updated:
- FavoritesEditModal, FolderEditModal, FolderSettingsModal
- LocalLibraryTagEditorModal, MigrationModal, PlaylistImportModal
- PlaylistModal, Sidebar, LocalLibraryView, PlaylistManagerView