feat(ui): bump thumbnail sizes 1.5x, make Settings fluid, add subscription_count stat

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_settings.py

@@ -76,7 +76,7 @@ async def test_system_stats_shape(client):
     resp = await client.get("/api/system/stats")
     assert resp.status_code == 200
     body = await resp.get_json()
-    for key in ("total_images", "total_tags", "storage_bytes", "tasks", "active_batch"):
+    for key in ("total_images", "total_tags", "storage_bytes", "subscription_count", "tasks", "active_batch"):
         assert key in body
     for status in ("pending", "queued", "processing", "complete", "skipped", "failed"):
         assert status in body["tasks"]