Add image/video background support across 5 layout layers (page, sidebar,
header, card, dialog) with per-layer effect controls and custom CSS injection.
- IndexedDB asset store for blob persistence (idb)
- AssetStoreProvider for blob URL lifecycle management
- BackgroundLayer component with CSS effects and prefers-reduced-motion support
- useBackground hook with inherit logic
- BackgroundUploader with local file and remote URL support
- BackgroundEffectsControls and ComponentCSSEditor UI components
- Background settings integrated into AppearanceTab in settings.tsx
- Layout, Card, and Dialog integration via non-breaking wrapper components
- Task 10: Bulk migrate CSS variable references from --primary/--secondary/etc to --color-primary/--color-secondary/etc across all components
- Task 11: Adapt CodeEditor to auto-detect theme from ThemeProvider context, remove hardcoded theme='dark' from plugin-config and bot config pages
