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/schema.graphql

@@ -14,6 +14,7 @@ type Query {
   giteaFeed: GiteaFeedItem
   steamStatus: SteamStatus
   me: User
+  bookmarks: [Bookmark!]!
   jobApplications: [JobApplication!]!
   jobApplication(id: ID!): JobApplication
 }
@@ -32,5 +33,7 @@ type Mutation {
   createActivity(input: CreateActivityInput!): Activity!
   createJobApplication(input: CreateJobApplicationInput!): JobApplication!
   updateJobApplication(id: ID!, input: UpdateJobApplicationInput!): JobApplication!
+  createBookmark(input: CreateBookmarkInput!): Bookmark!
+  deleteBookmark(id: ID!): Bookmark!
   deleteJobApplication(id: ID!): Boolean!
 }