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/collections.go

@@ -75,14 +75,15 @@ type BookInfo struct {
 }
 
 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"`
+	ID           string     `json:"id"`
+	CollectionID string     `json:"collection_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 {