fix(desktop): add missing PATCH /api/sessions/{id} so rename works (#36249)

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.

apps/desktop/src/store/notifications.ts

@@ -68,7 +68,8 @@ const ERROR_SUMMARIES: { test: (msg: string) => boolean; summarize: (msg: string
   },
   {
     test: msg => /method not allowed/i.test(msg),
-    summarize: () => 'The desktop backend does not support that audio endpoint yet. Restart Hermes Desktop.'
+    summarize: () =>
+      'The desktop backend rejected that request (405 Method Not Allowed). Try restarting Hermes Desktop.'
   },
   {
     test: msg => /microphone permission/i.test(msg),