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/schema/bookmark.graphql

@@ -0,0 +1,14 @@
+type Bookmark {
+  id: ID!
+  createdAt: Time!
+  updatedAt: Time!
+  category: String!
+  name: String!
+  link: String!
+}
+
+input CreateBookmarkInput {
+  category: String!
+  name: String!
+  link: String!
+}