feat(dashboard): implement Phase 6 TypeScript type definitions

Add TypeScript interfaces for Carousel-style dashboard to api.d.ts:

New Interfaces:
1. SectionData
   - Matches handlers.SectionData in collections.go (lines 73-81)
   - id: Collection name (string)
   - is_system: Boolean flag (true for system collections, false for user)
   - title: Display title
   - description: Collection description
   - icon: Emoji icon
   - items: Array of BookInfo objects
   - view_all_url: URL to view all items (system collections only)
   - priority: Display order (lower numbers first)

2. DashboardPreferences
   - Matches database.UserDashboardPreferences (models.go:381-390)
   - library_id: Library UUID
   - hidden_collections: Array of collection names to hide
   - collection_order: Array of collection names for custom ordering
   - items_per_section: Number of items per section

Existing Interface:
- BookInfo: Already defined (media_item_id, title, author, cover_image_path)
  - Reused by SectionData for items array
  - No duplicate definitions needed

Key Compliance:
- is_system: boolean matches database is_system_collection field
- media_item_id matches Go BookInfo.MediaItemID field
- Uses existing BookInfo struct (no duplicates)
- Added to existing api.d.ts file (follows established pattern)

web/src/types/api.d.ts

@@ -97,6 +97,31 @@ export interface BookInfo {
     cover_image_path: string;
 }
 
+// Dashboard type definitions
+// CRITICAL: Must match Go handler return types EXACTLY
+// Source: handlers.SectionData in collections.go (lines 73-81)
+// Used in: dashboard API responses, TypeScript dashboard components
+export interface SectionData {
+    id: string;
+    is_system: boolean;
+    title: string;
+    description: string;
+    icon: string;
+    items: BookInfo[];
+    view_all_url: string;
+    priority: number;
+}
+
+// Matches database.UserDashboardPreferences and dashboard preferences API
+// Source: internal/database/models.go:381-390
+// Used in: dashboard preferences API
+export interface DashboardPreferences {
+    library_id: string;
+    hidden_collections: string[];
+    collection_order: string[];
+    items_per_section: number;
+}
+
 // Matches handlers.UnlinkedBookData JSON response
 // Used in: unlinked_books.ts, unlinked_books.templ
 export interface UnlinkedBookData {