fix: delete activity removes it from index.json; detail page uses lazy load

delete_activity now updates data_dir/index.json so merge_all no longer
re-adds the summary for a deleted activity, preventing the broken
"Activity not found" state after deletion.

ActivityDetailLoader switches from loadIndex (all year shards) to
loadIndexPaged (first year shard only) + direct file fallback, so
opening an activity detail page no longer downloads the entire history.

bincio/serve/server.py

@@ -1288,6 +1288,17 @@ async def delete_activity(
     if images_dir.exists():
         shutil.rmtree(images_dir)
 
+    # Remove from the extract-level flat index so merge_all doesn't re-add
+    # the summary even though the detail file is gone.
+    index_path = dd / "index.json"
+    if index_path.exists():
+        try:
+            idx = json.loads(index_path.read_text(encoding="utf-8"))
+            idx["activities"] = [a for a in idx.get("activities", []) if a.get("id") != activity_id]
+            index_path.write_text(json.dumps(idx, indent=2, ensure_ascii=False))
+        except Exception:
+            pass  # corrupt index — merge_all will clean up on next run
+
     # Remove from dedup cache so the file can be re-uploaded if needed
     cache_path = dd / ".bincio_cache.json"
     if cache_path.exists():