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/credentials.ts

@@ -1,5 +1,3 @@
-import type { StoredCredential } from "./types";
-
 /**
  * Derives a credential key from a remote URL, scoped to host + organisation —
  * the same granularity Azure DevOps / GitHub use. Examples:
@@ -37,13 +35,6 @@ export function orgKeyFromUrl(raw: string): string | null {
   return org ? `${host}/${org}` : host;
 }
 
-/** A stored credential is expired only if it carries a past expiry date. */
-export function isCredentialExpired(cred: StoredCredential): boolean {
-  if (!cred.expiresAt) return false;
-  const time = new Date(cred.expiresAt).getTime();
-  return !Number.isNaN(time) && time < Date.now();
-}
-
 const AUTH_PREFIX = "AUTH_FAILED:";
 
 export function isAuthError(message: string): boolean {