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/git.ts

@@ -381,13 +381,8 @@ export function credLoad(key: string): Promise<StoredCredential | null> {
   return invoke<StoredCredential | null>("cred_load", { key });
 }
 
-export function credSave(
-  key: string,
-  username: string,
-  password: string,
-  expiresAt: string | null,
-): Promise<void> {
-  return invoke<void>("cred_save", { key, username, password, expiresAt });
+export function credSave(key: string, username: string, password: string): Promise<void> {
+  return invoke<void>("cred_save", { key, username, password });
 }
 
 export function credDelete(key: string): Promise<void> {