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/hooks/use-background.ts

@@ -0,0 +1,26 @@
+import { useTheme } from '@/components/use-theme'
+
+import type { BackgroundConfig } from '@/lib/theme/tokens'
+import { defaultBackgroundConfig } from '@/lib/theme/tokens'
+
+type BackgroundLayerId = 'page' | 'sidebar' | 'header' | 'card' | 'dialog'
+
+/**
+ * 获取指定层级的背景配置
+ * 处理继承逻辑：如果 inherit 为 true，返回页面级别配置
+ * @param layerId - 背景层级标识
+ * @returns 对应层级的背景配置
+ */
+export function useBackground(layerId: BackgroundLayerId): BackgroundConfig {
+  const { themeConfig } = useTheme()
+  const bgMap = themeConfig.backgroundConfig ?? {}
+
+  const config = bgMap[layerId] ?? defaultBackgroundConfig
+
+  // 处理继承逻辑：非 page 层级且 inherit 为 true，返回 page 配置
+  if (layerId !== 'page' && config.inherit) {
+    return bgMap.page ?? defaultBackgroundConfig
+  }
+
+  return config
+}