feat(ui): default credential dialog to save session

The credential dialog now starts with session saving enabled by
default, reducing extra user interaction for the common case. This
improves usability while keeping the option available to change.

- Set saveSession default to true in CredentialDialog

src/lib/components/CredentialDialog.svelte

@@ -35,7 +35,7 @@
   let username = $state("");
   let password = $state("");
   let showPassword = $state(false);
-  let saveSession = $state(false);
+  let saveSession = $state(true);
   let expiresAt = $state("");
 
   let canSubmit = $derived(