refactor(credentials): remove credential expiry support and API

The credential expiry feature has been removed from both the Rust
backend and the frontend. Stored credentials now include only
username and password.

- Remove expiresAt field from StoredCredential
- Simplify API by removing expiry param from credSave
- Drop expiry UI and expiry checks across the app

src/lib/components/AiSettingsDialog.svelte

@@ -85,7 +85,7 @@
     const key = CRED_KEYS[target];
     const trimmed = value.trim();
     if (trimmed) {
-      await credSave(key, "api-key", trimmed, null);
+      await credSave(key, "api-key", trimmed);
     } else {
       await credDelete(key);
     }