Add database-backed bookmarks via GraphQL

Replace hardcoded bookmarks in the frontend with a GORM-backed Bookmark
model exposed through GraphQL query and admin-only create/delete mutations.
Frontend groups bookmarks by category dynamically from the store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/graph/model/models_gen.go

@@ -18,6 +18,12 @@ type CreateActivityInput struct {
 	Link *string `json:"link,omitempty"`
 }
 
+type CreateBookmarkInput struct {
+	Category string `json:"category"`
+	Name     string `json:"name"`
+	Link     string `json:"link"`
+}
+
 type CreateFavoriteInput struct {
 	Type string  `json:"type"`
 	Name string  `json:"name"`