feat: Add collection detail page with /collections/:id route

Add comprehensive collection detail page that works for both system collections
(continue-reading, recently-added, not-started) and user collections.

Backend changes:
- Add new /collections/:id route in internal/router/frontend.go
  - Fetches collection using GetCollection with UUID parameter
  - Determines collection type from QueryType field
  - Resolves library_id for system collections
  - Converts database.MediaItems to handlers.BookInfo for display
  - Renders CollectionDetail template with collection and books data

- Update SectionData struct in internal/handlers/collections.go
  - Add CollectionID string field for view all links

- Update BuildSections() in internal/handlers/dashboard.go
  - Pass CollectionID to SectionData for proper link generation

- Simplify getViewAllURL() in internal/handlers/dashboard.go
  - Return /collections/{collectionID} instead of /section/{type}
  - Works uniformly for both system and user collections

Frontend changes:
- Fix CollectionDetail template in templates/collections.templ
  - Fix broken div nesting causing compilation error
  - Add null check for CoverImagePath to prevent broken images
  - Update aspect ratio to modern aspect-[3/4] syntax
  - Use responsive widths (w-16 sm:w-20) for mobile/desktop
  - Improve card layout with horizontal flex structure
  - Add placeholder image fallback for books without covers
  - Remove erroneous renderBooks() function call

This change aligns with the backend update where system collections are
now pre-made user collections in the database with query_type fields.
All collections can now use the same CollectionDetail template for a
consistent viewing experience.

internal/handlers/dashboard.go

@@ -40,6 +40,9 @@ func (h *DashboardHandler) GetSections(c echo.Context) error {
 	prefs, _ := h.dashboardService.GetDashboardPreferences(c.Request().Context(), userUUID, libUUID)
 
 	limit := 20
+	if prefs.ItemsPerSection.Valid {
+		limit = int(prefs.ItemsPerSection.Int32)
+	}
 	if limitStr := c.QueryParam("limit"); limitStr != "" {
 		if l, err := strconv.Atoi(limitStr); err == nil && l > 0 && l <= 100 {
 			limit = l
@@ -83,6 +86,11 @@ func (h *DashboardHandler) UpdatePreferences(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
 	}
 
+	// Sanitize preferences (remove duplicates)
+	cleanHidden, cleanOrder := h.dashboardService.SanitizeDashboardPreferences(req.HiddenCollections, req.CollectionOrder)
+	req.HiddenCollections = cleanHidden
+	req.CollectionOrder = cleanOrder
+
 	prefs, err := h.dashboardService.UpsertDashboardPreferences(c.Request().Context(), database.UpsertDashboardPreferencesParams{
 		UserID:            pgtype.UUID{Bytes: userUUID, Valid: true},
 		LibraryID:         pgtype.UUID{Bytes: libUUID, Valid: true},
@@ -157,29 +165,49 @@ func BuildSections(sections []services.DashboardSection) []SectionData {
 		}
 
 		result = append(result, SectionData{
-			ID:          ds.CollectionName,
-			IsSystem:    ds.IsSystem,
-			Title:       ds.Title,
-			Description: ds.Description,
-			Icon:        ds.Icon,
-			Items:       bookCards,
-			ViewAllURL:  getViewAllURL(ds.CollectionName, ds.QueryType),
-			Priority:    ds.Priority,
+			ID:           ds.CollectionName,
+			CollectionID: ds.CollectionID.String(),
+			IsSystem:     ds.IsSystem,
+			Title:        ds.Title,
+			Description:  ds.Description,
+			Icon:         ds.Icon,
+			Items:        bookCards,
+			ViewAllURL:   getViewAllURL(ds.CollectionID.String()),
+			Priority:     ds.Priority,
 		})
 	}
 
 	return result
 }
 
-func getViewAllURL(key, queryType string) string {
-	urls := map[string]string{
-		"continue-reading": "/section/continue-reading",
-		"recently-added":   "/section/recently-added",
-		"recently-read":    "/history",
-		"not-started":      "/section/not-started",
-	}
-	if url, exists := urls[queryType]; exists {
-		return url
+func getViewAllURL(collectionID string) string {
+	if collectionID != "" {
+		return "/collections/" + collectionID
 	}
 	return ""
 }
+
+func (h *DashboardHandler) GetPreferences(c echo.Context) error {
+	user := c.Get("user").(database.Users)
+	userUUID := uuid.UUID(user.ID.Bytes)
+	libraryID := c.QueryParam("library_id")
+
+	// ✅ Add validation
+	if libraryID == "" {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "library_id required"})
+	}
+
+	libUUID, err := uuid.Parse(libraryID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+	}
+	prefs, err := h.dashboardService.GetDashboardPreferences(c.Request().Context(), userUUID, libUUID)
+	if err != nil {
+		return c.JSON(http.StatusNotFound, map[string]string{"error": "Preferences not found"})
+	}
+	return c.JSON(http.StatusOK, map[string]interface{}{
+		"hidden_collections": prefs.HiddenCollections,
+		"collection_order":   prefs.CollectionOrder,
+		"items_per_section":  prefs.ItemsPerSection.Int32,
+	})
+}