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);
     }

src/lib/components/CredentialDialog.svelte

@@ -17,7 +17,7 @@
     action: "push" | "pull" | "fetch" | "clone";
     error: string;
     isBusy: boolean;
-    onSubmit: (username: string, password: string, save: boolean, expiresAt: string | null) => void;
+    onSubmit: (username: string, password: string, save: boolean) => void;
     onCancel: () => void;
   }
 
@@ -36,7 +36,6 @@
   let password = $state("");
   let showPassword = $state(false);
   let saveSession = $state(true);
-  let expiresAt = $state("");
 
   let canSubmit = $derived(
     !isBusy &&
@@ -62,12 +61,7 @@
   function handleSubmit(e: SubmitEvent) {
     e.preventDefault();
     if (!canSubmit) return;
-    onSubmit(
-      mode === "token" ? "oauth2" : username,
-      password,
-      saveSession,
-      saveSession && expiresAt ? expiresAt : null,
-    );
+    onSubmit(mode === "token" ? "oauth2" : username, password, saveSession);
   }
 </script>
 
@@ -191,19 +185,6 @@
         </div>
       {/if}
 
-      {#if saveSession}
-        <div class="cred-expiry">
-          <label class="cred-field-label" for="cred-expiry">Expiration date (optional)</label>
-          <input
-            id="cred-expiry"
-            type="date"
-            bind:value={expiresAt}
-            disabled={isBusy}
-          />
-          <span class="cred-expiry-hint">After this date you'll automatically be asked to log in again.</span>
-        </div>
-      {/if}
-
       <div class="cred-footer">
         <label class="cred-save">
           <input type="checkbox" bind:checked={saveSession} disabled={isBusy} />