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/spotify_helpers.go

@@ -0,0 +1,51 @@
+package graph
+
+import (
+	"adam-french.co.uk/backend/graph/model"
+	"github.com/zmb3/spotify/v2"
+)
+
+func mapSpotifyImages(images []spotify.Image) []*model.SpotifyImage {
+	result := make([]*model.SpotifyImage, len(images))
+	for i, img := range images {
+		result[i] = &model.SpotifyImage{URL: img.URL}
+	}
+	return result
+}
+
+func mapSpotifyTrack(track *spotify.FullTrack) *model.SpotifyTrack {
+	artists := make([]*model.SpotifyArtist, len(track.Artists))
+	for i, a := range track.Artists {
+		artists[i] = &model.SpotifyArtist{Name: a.Name}
+	}
+	return &model.SpotifyTrack{
+		Name:    track.Name,
+		Artists: artists,
+		Album: &model.SpotifyAlbum{
+			Name:   track.Album.Name,
+			Images: mapSpotifyImages(track.Album.Images),
+		},
+	}
+}
+
+func mapRecentItems(items []spotify.RecentlyPlayedItem) []*model.SpotifyRecentItem {
+	result := make([]*model.SpotifyRecentItem, len(items))
+	for i, item := range items {
+		artists := make([]*model.SpotifyArtist, len(item.Track.Artists))
+		for j, a := range item.Track.Artists {
+			artists[j] = &model.SpotifyArtist{Name: a.Name}
+		}
+		result[i] = &model.SpotifyRecentItem{
+			PlayedAt: item.PlayedAt,
+			Track: &model.SpotifyTrack{
+				Name:    item.Track.Name,
+				Artists: artists,
+				Album: &model.SpotifyAlbum{
+					Name:   item.Track.Album.Name,
+					Images: mapSpotifyImages(item.Track.Album.Images),
+				},
+			},
+		}
+	}
+	return result
+}