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/types.ts

@@ -310,5 +310,4 @@ export interface ReflogEntry {
 export interface StoredCredential {
   username: string;
   password: string;
-  expiresAt?: string | null;
 }