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/lib/devices/device_detail.dart

@@ -4,6 +4,7 @@ import 'package:go_router/go_router.dart';
 
 import '../model/device.dart';
 import '../model/device_type.dart';
+import '../routes.dart';
 import '../utils/friendly_date_formatter.dart';
 import '../utils/oott_api.dart';
 import '../widgets/status_badge.dart';
@@ -220,41 +221,57 @@ class _DeviceActions extends StatelessWidget {
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
 
+    final destructiveStyle = TextButton.styleFrom(
+      foregroundColor: colorScheme.error,
+    );
+
     if (device.isRegistered) {
-      return Wrap(
-        spacing: 12,
-        runSpacing: 12,
+      return Row(
         children: [
           FilledButton.icon(
             onPressed: () => showEditDeviceDialog(context, device, onAction),
             icon: const Icon(Icons.edit),
             label: const Text('Edit'),
           ),
-          OutlinedButton.icon(
-            style: OutlinedButton.styleFrom(
-              foregroundColor: colorScheme.error,
-              side: BorderSide(color: colorScheme.error),
+          const Spacer(),
+          TextButton.icon(
+            style: destructiveStyle,
+            onPressed: () => confirmForgetDevice(
+              context,
+              device,
+              onAction,
+              onDeleted: () => context.go(Routes.devices),
             ),
-            onPressed: () => confirmForgetDevice(context, device, onAction),
             icon: const Icon(Icons.link_off),
             label: const Text('Forget Device'),
           ),
         ],
       );
     }
-    return Wrap(
-      spacing: 12,
-      runSpacing: 12,
+    return Row(
       children: [
         FilledButton.icon(
           onPressed: () => showRegisterDeviceDialog(context, device, onAction),
           icon: const Icon(Icons.how_to_reg),
           label: const Text('Register Device'),
         ),
-        OutlinedButton.icon(
+        const SizedBox(width: Insets.md),
+        TextButton.icon(
           onPressed: () => showDeviceIdentificationDialog(context, device),
           icon: const Icon(Icons.help_outline),
-          label: const Text('How to identify this device'),
+          label: const Text('How to identify'),
+        ),
+        const Spacer(),
+        TextButton.icon(
+          style: destructiveStyle,
+          onPressed: () => confirmDeleteDevice(
+            context,
+            device,
+            onAction,
+            onDeleted: () => context.go(Routes.devices),
+          ),
+          icon: const Icon(Icons.delete_outline),
+          label: const Text('Delete'),
         ),
       ],
     );