feat(dashboard): add background customization system

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

dashboard/package.json

@@ -60,6 +60,7 @@
     "dagre": "^0.8.5",
     "date-fns": "^4.1.0",
     "html-to-image": "^1.11.13",
+    "idb": "^8.0.3",
     "jotai": "^2.16.0",
     "katex": "^0.16.27",
     "lucide-react": "^0.556.0",