feat(ui): trap focus in the Settings dialog (T7.5, FR-UX-4)

src/lib/views/Settings.svelte

@@ -8,6 +8,7 @@
   import ConsentNotice from "../components/ConsentNotice.svelte";
   import { open } from "@tauri-apps/plugin-dialog";
   import type { BackendId, SyncKind, SyncTargetConfig } from "../api";
+  import { trapFocus } from "../actions/trapFocus";
 
   let { onClose }: { onClose: () => void } = $props();
 
@@ -125,7 +126,14 @@
   }
 </script>
 
-<div class="overlay" role="dialog" aria-modal="true" aria-label="Settings">
+<div
+  class="overlay"
+  role="dialog"
+  aria-modal="true"
+  aria-label="Settings"
+  tabindex="-1"
+  use:trapFocus
+>
   <div class="panel">
     <header>
       <strong>Settings</strong>