fix(ui): logout type error when user is null during locale switch (#13514)

### What?

Prevent a `TypeError: Cannot read properties of null (reading
'collection')` in the admin UI when switching locales by hardening
`AuthProvider.logOut`.

### Why?

During locale transitions, user can briefly be null. The existing code
used `user.collection` unguarded

### How?

- Use `userSlug` over `user.collection`.
- Always clear local auth in a `finally` block (`setNewUser(null)`,
`revokeTokenAndExpire()`), regardless of request outcome.

Fixes #13313 


---
- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1211093549155962

packages/ui/src/providers/Auth/index.tsx

@@ -192,13 +192,17 @@ export function AuthProvider({
 
   const logOut = useCallback(async () => {
     try {
-      await requests.post(`${serverURL}${apiRoute}/${user.collection}/logout`)
-      setNewUser(null)
-      revokeTokenAndExpire()
+      if (user && user.collection) {
+        await requests.post(`${serverURL}${apiRoute}/${user.collection}/logout`)
+      }
       return true
     } catch (e) {
       toast.error(`Logging out failed: ${e.message}`)
       return false
+    } finally {
+      // Always clear local auth state
+      setNewUser(null)
+      revokeTokenAndExpire()
     }
   }, [apiRoute, revokeTokenAndExpire, serverURL, setNewUser, user])