test(ui-lab): add extreme mock scenario

Add an extreme UI Lab scenario with virtual-scale library, album, cluster, and tag counts while keeping the rendered media fixture set manageable.

Return the full extreme tag set from the mock backend so tag manager layouts can be stress-tested with 10k-100k tag counts.

src/dev/mockBackend.ts

@@ -206,7 +206,7 @@ function updateImages(ids: number[], updates: { favorite?: boolean | null; ratin
 function refreshAlbumCounts() {
   for (const album of db.albums) {
     const ids = db.albumImageIds[album.id] ?? [];
-    album.image_count = ids.length;
+    if (db.scenario !== "extreme") album.image_count = ids.length;
     album.cover_image_id = ids[0] ?? null;
     album.cover_thumbnail_path = db.images.find((image) => image.id === ids[0])?.thumbnail_path ?? null;
     album.updated_at = new Date().toISOString();
@@ -317,7 +317,11 @@ export async function handleMockCommand(cmd: string, payload?: unknown): Promise
     case "get_visual_clusters":
       return db.scenario === "empty" ? [] : db.visualClusters;
     case "get_explore_tags":
-      return db.scenario === "empty" ? [] : db.exploreTags.slice(0, Number(p.limit ?? 180));
+      return db.scenario === "empty"
+        ? []
+        : db.scenario === "extreme"
+          ? db.exploreTags
+          : db.exploreTags.slice(0, Number(p.limit ?? 180));
     case "get_related_tags": {
       const relatedCounts = new Map<string, number>();
       for (const imageTags of Object.values(db.tagsByImageId)) {