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/src/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from 'react-dom/client'
 import { RouterProvider } from '@tanstack/react-router'
 import './index.css'
 import { router } from './router'
+import { AssetStoreProvider } from './components/asset-provider'
 import { ThemeProvider } from './components/theme-provider'
 import { AnimationProvider } from './components/animation-provider'
 import { TourProvider, TourRenderer } from './components/tour'
@@ -12,15 +13,17 @@ import { ErrorBoundary } from './components/error-boundary'
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <ErrorBoundary>
-      <ThemeProvider defaultTheme="system">
-        <AnimationProvider>
-          <TourProvider>
-            <RouterProvider router={router} />
-            <TourRenderer />
-            <Toaster />
-          </TourProvider>
-        </AnimationProvider>
-      </ThemeProvider>
+      <AssetStoreProvider>
+        <ThemeProvider defaultTheme="system">
+          <AnimationProvider>
+            <TourProvider>
+              <RouterProvider router={router} />
+              <TourRenderer />
+              <Toaster />
+            </TourProvider>
+          </AnimationProvider>
+        </ThemeProvider>
+      </AssetStoreProvider>
     </ErrorBoundary>
   </StrictMode>
 )