feat(web): cloud chat-model selector in Settings

Pick which OpenAI model answers on the Cloud backend (gpt-4o / -mini / 4.1 /
4.1-mini / o4-mini, or Default). Persisted in localStorage, sent as `model` in
the chat request; respond() applies it only on the cloud backend (local/mi50
keep their fixed models). Reachable from desktop + mobile via Settings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/chat.py

@@ -113,14 +113,21 @@ def build_messages(session_id: str, user_msg: str) -> list[Message]:
     return messages
 
 
-def respond(session_id: str, user_msg: str, backend: Backend = "cloud") -> str:
-    """Produce Lyra's reply to a single user message and persist the exchange."""
+def respond(session_id: str, user_msg: str, backend: Backend = "cloud",
+            model_override: str | None = None) -> str:
+    """Produce Lyra's reply to a single user message and persist the exchange.
+
+    `model_override` (from the UI's cloud-model picker) only applies on the cloud
+    backend; local/mi50 keep their own configured models.
+    """
     cfg = config.load()
     # Live chat uses the stronger chat_model on cloud (bulk consolidation keeps
     # cloud_model). local/mi50 use their own configured model.
     model = {"local": cfg.local_model, "cloud": cfg.chat_model, "mi50": cfg.mi50_model}.get(
         backend, backend
     )
+    if model_override and backend == "cloud":
+        model = model_override
     logbus.log(
         "info", "chat request", session=session_id, backend=backend,
         model=model, embed=cfg.embed_backend,