Consolidate frontend REST calls with GraphQL

Replace 5 separate REST calls on home page load with a single GraphQL
query. Add homeData store that fetches posts, favorites, activities,
spotify, and auth in one request. Convert all admin mutations and
auth flows to use GraphQL. Add album images to Spotify GraphQL schema.

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

backend/graph/schema/auth.graphql

@@ -0,0 +1,8 @@
+input LoginInput {
+  username: String!
+  password: String!
+}
+
+type AuthPayload {
+  user: User!
+}