feat(webui): polish chat layout and titles

Align the WebUI sidebar and chat chrome with the updated design, and generate WebUI session titles asynchronously without blocking turns.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/agent/test_session_manager_history.py

@@ -1,4 +1,4 @@
-from nanobot.session.manager import Session
+from nanobot.session.manager import Session, SessionManager
 
 
 def _assert_no_orphans(history: list[dict]) -> None:
@@ -31,6 +31,18 @@ def _tool_turn(prefix: str, idx: int) -> list[dict]:
     ]
 
 
+def test_list_sessions_includes_metadata_title(tmp_path):
+    manager = SessionManager(tmp_path)
+    session = manager.get_or_create("websocket:chat-title")
+    session.metadata["title"] = "自动生成标题"
+    manager.save(session)
+
+    rows = manager.list_sessions()
+
+    assert rows[0]["key"] == "websocket:chat-title"
+    assert rows[0]["title"] == "自动生成标题"
+
+
 # --- Original regression test (from PR 2075) ---
 
 def test_get_history_drops_orphan_tool_results_when_window_cuts_tool_calls():