feat(collections): add helper functions for template rendering

Add data retrieval helpers for SSR template rendering:
- GetDeviceMappingsData: Fetch device shelf mappings for device settings UI
- GetUserCollectionsList: Get all user collections for dropdowns and listings
- GetCollectionData: Get single collection with metadata
- GetCollectionBooksData: Get books in a collection

These functions support the Phase 9 frontend features by providing
efficient data access for template rendering without modifying
existing API endpoints.

internal/handlers/collections.go

@@ -487,3 +487,13 @@ func (h *CollectionHandler) GetCollectionData(c echo.Context, collectionID uuid.
 func (h *CollectionHandler) GetCollectionBooksData(c echo.Context, collectionID uuid.UUID) ([]database.GetCollectionItemsRow, error) {
 	return h.collectionService.GetCollectionBooks(c.Request().Context(), collectionID)
 }
+
+func (h *CollectionHandler) GetDeviceMappingsData(c echo.Context, deviceID uuid.UUID) ([]database.GetDeviceShelfMappingsRow, error) {
+	return h.collectionService.GetDeviceShelfMappings(c.Request().Context(), deviceID)
+}
+
+func (h *CollectionHandler) GetUserCollectionsList(c echo.Context) ([]database.Collections, error) {
+	user := c.Get("user").(database.Users)
+	userUUID := uuid.UUID(user.ID.Bytes)
+	return h.collectionService.GetUserCollections(c.Request().Context(), userUUID)
+}