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/components/asset-provider.tsx

@@ -0,0 +1,64 @@
+import { createContext, useContext, useEffect, useMemo, useRef } from 'react'
+import type { ReactNode } from 'react'
+
+import { getAsset } from '@/lib/asset-store'
+
+type AssetStoreContextType = {
+  getAssetUrl: (assetId: string) => Promise<string | undefined>
+}
+
+const AssetStoreContext = createContext<AssetStoreContextType | null>(null)
+
+type AssetStoreProviderProps = {
+  children: ReactNode
+}
+
+export function AssetStoreProvider({ children }: AssetStoreProviderProps) {
+  const urlCache = useRef<Map<string, string>>(new Map())
+
+  const getAssetUrl = async (assetId: string): Promise<string | undefined> => {
+    // Check cache first
+    const cached = urlCache.current.get(assetId)
+    if (cached) {
+      return cached
+    }
+
+    // Fetch from IndexedDB
+    const record = await getAsset(assetId)
+    if (!record) {
+      return undefined
+    }
+
+    // Create blob URL and cache it
+    const url = URL.createObjectURL(record.blob)
+    urlCache.current.set(assetId, url)
+    return url
+  }
+
+  const value = useMemo(
+    () => ({
+      getAssetUrl,
+    }),
+    [],
+  )
+
+  // Cleanup: revoke all blob URLs on unmount
+  useEffect(() => {
+    return () => {
+      urlCache.current.forEach((url) => {
+        URL.revokeObjectURL(url)
+      })
+      urlCache.current.clear()
+    }
+  }, [])
+
+  return <AssetStoreContext.Provider value={value}>{children}</AssetStoreContext.Provider>
+}
+
+export function useAssetStore() {
+  const context = useContext(AssetStoreContext)
+  if (!context) {
+    throw new Error('useAssetStore must be used within AssetStoreProvider')
+  }
+  return context
+}