fix(settings): include current mode in fallback availableModes

When v2_available_titlebar_modes fails, the dropdown's selected value
may not appear in its options. Include the user's current titlebarMode
in the fallback list to ensure the selected value is always present.

cc

src/lib/components/views/SettingsView.svelte

@@ -1006,7 +1006,12 @@
 
   // Title bar settings
   let titlebarMode = $state<TitlebarMode>(getMode());
-  let availableModes = $state<TitlebarMode[]>(['qbz', 'system', 'hidden']);
+  // Fallback list before v2_available_titlebar_modes resolves. Always include
+  // the currently active mode so the dropdown shows the user's selected value
+  // even when the backend invoke fails.
+  let availableModes = $state<TitlebarMode[]>(
+    titlebarMode === 'plasma' ? ['qbz', 'system', 'plasma', 'hidden'] : ['qbz', 'system', 'hidden']
+  );
   let isSandboxed = $state(false);
   let matchSystemWindowChromeState = $state(getMatchSystemWindowChrome());
   let windowControlsVisible = $state(getShowWindowControls());