The desktop rename dialog sent PATCH /api/sessions/{id}, but the backend
only defined GET and DELETE for that path — FastAPI returned 405 Method
Not Allowed, surfaced to the user as "Rename failed". Add the PATCH route
backed by SessionDB.set_session_title (handles sanitization, uniqueness,
and clearing the title when empty).
Also fix a misleading notification: any 405 was summarized as an unrelated
"does not support that audio endpoint" message. Make it a generic 405 hint.
7fbe9b79ab