feat(dashboard): implement Phase 4 API handlers for Carousel-style dashboard

Add dashboard API endpoints with handler layer:

Step 1: Add SectionData to collections.go
- SectionData struct represents dashboard section (carousel of books)
- Used by: Dashboard handler, Templates (SSR), API JSON responses
- Shared type from collections.go (no duplicate definitions)
- Fields: ID, IsSystem, Title, Description, Icon, Items, ViewAllURL, Priority

Step 2: Create dashboard.go handler
- DashboardHandler struct with injected database and dashboard service
- GetSections: Returns dashboard sections as JSON (mobile apps, web UI TypeScript, plugins)
  * Validates library_id parameter
  * Fetches user dashboard preferences
  * Configurable limit (default 20, max 100)
  * Calls service layer for business logic
  * Converts service types to handler types for JSON serialization
- UpdatePreferences: Saves dashboard preferences
  * Validates library_id
  * Upserts user dashboard preferences
- RestoreSystemCollection: Resets system collection to defaults
  * Validates collection_name against allowed system collections
  * Deletes user's copy (system collection reappears automatically)
- BuildSections: Converts service DashboardSection to handler SectionData
  * Converts database.MediaItems to handlers.BookInfo
  * Uses shared types from collections.go
- getViewAllURL: Maps system collections to their view-all URLs
- Reuses existing textToString helper from collections.go

Architecture Compliance:
- Generic API handler for reuse by SSR, mobile, plugins
- Uses shared types from collections.go (SectionData, BookInfo)
- IsSystem bool matches database field (no string conversion)
- Single service method returns structured data (simpler, less bugs)
- Handler just converts types (no matching logic needed)
- Reusable by mobile apps, web UI, plugins

internal/handlers/collections.go

@@ -70,6 +70,17 @@ type BookInfo struct {
 	CoverImagePath string `json:"cover_image_path"`
 }
 
+type SectionData struct {
+	ID          string     `json:"id"`
+	IsSystem    bool       `json:"is_system"`
+	Title       string     `json:"title"`
+	Description string     `json:"description"`
+	Icon        string     `json:"icon"`
+	Items       []BookInfo `json:"items"`
+	ViewAllURL  string     `json:"view_all_url"`
+	Priority    int        `json:"priority"`
+}
+
 func (h *CollectionHandler) CreateCollection(c echo.Context) error {
 	user := c.Get("user").(database.Users)
 	userUUID := uuid.UUID(user.ID.Bytes)