Add permanent device deletion and refine frontend UI/snackbars

Backend:
- Add db::devices::delete to erase a device and its events atomically
- Expose DELETE /api/devices/{mac}/permanently, wired to OpenAPI
- Cover the new db method and endpoint with tests

Frontend:
- Delete action for not-registered devices (detail screen + list row)
  and an opt-in "permanently delete" checkbox in the Forget dialog
- Navigate to the devices list after deleting from the detail screen
- Refine button emphasis to M3: single filled primary, error-colored
  text buttons for destructive actions, Test demoted to filled-tonal
- Flash the backend-config Test button red on a failed connection test
- Render snackbars through a top-level ScaffoldMessenger host so they
  show above dialogs; keep the built-in SnackBar (with an Overlay host)

Docs:
- CLAUDE.md: rustfmt edition 2024, don't revert formatter-only changes,
  prefer built-in Flutter components, follow existing patterns + M3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/snackbar_host_test.dart

@@ -0,0 +1,66 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/theme/catppuccin_mocha_theme.dart';
+import 'package:frontend/utils/ui_snackbars.dart';
+
+import '../helpers/pump_app.dart';
+
+void main() {
+  // Builds an app wired exactly like main.dart: the router/Navigator sits under
+  // the snackbar host so snackbars render above dialogs.
+  Future<BuildContext> pumpHostedApp(WidgetTester tester) async {
+    late BuildContext pageContext;
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: catppuccinMochaDarkTheme,
+        builder: buildSnackbarHost,
+        home: Builder(
+          builder: (context) {
+            pageContext = context;
+            return const Scaffold(body: SizedBox.expand());
+          },
+        ),
+      ),
+    );
+    return pageContext;
+  }
+
+  testWidgets('snackbar shows through the host above an open dialog', (
+    tester,
+  ) async {
+    final context = await pumpHostedApp(tester);
+
+    // Open a dialog so a modal barrier is on screen.
+    showDialog<void>(
+      context: context,
+      builder: (_) => const AlertDialog(title: Text('A dialog')),
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('A dialog'), findsOneWidget);
+
+    UISnackbars.showSuccess(context, 'Hello there');
+    await tester.pumpAndSettle();
+
+    expect(find.byType(SnackBar), findsOneWidget);
+    expect(find.text('Hello there'), findsOneWidget);
+
+    await tearDownTree(tester);
+  });
+
+  testWidgets('the snackbar close-icon tooltip does not crash for lack of an '
+      'overlay', (tester) async {
+    final context = await pumpHostedApp(tester);
+
+    UISnackbars.showError(context, 'Boom');
+    await tester.pumpAndSettle();
+
+    // Long-pressing the close icon shows its tooltip, which needs an Overlay
+    // ancestor — the previous host had none and threw "No Overlay widget found".
+    await tester.longPress(find.byIcon(Icons.close));
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(tester.takeException(), isNull);
+
+    await tearDownTree(tester);
+  });
+}