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_actions.dart

@@ -6,20 +6,111 @@ import '../utils/oott_api.dart';
 import '../utils/ui_snackbars.dart';
 import '../theme/dimens.dart';
 
+/// Confirms forgetting (and optionally permanently deleting) a registered
+/// device.
+///
+/// On a successful permanent deletion [onDeleted] is invoked when provided
+/// (e.g. to navigate away from a now-stale detail page); otherwise [onRefresh]
+/// is used. A plain unregister always calls [onRefresh].
 Future<void> confirmForgetDevice(
   BuildContext context,
   Device device,
-  VoidCallback onRefresh,
-) async {
+  VoidCallback onRefresh, {
+  VoidCallback? onDeleted,
+}) async {
+  final colorScheme = Theme.of(context).colorScheme;
+  bool alsoDelete = false;
+
+  final action = await showDialog<bool>(
+    context: context,
+    builder: (context) => StatefulBuilder(
+      builder: (context, setDialogState) => AlertDialog(
+        title: const Text('Forget Device'),
+        content: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text(
+              'This device will be unregistered and will no longer be linked '
+              'to ${device.owner}. Are you sure?',
+            ),
+            const SizedBox(height: Insets.md),
+            CheckboxListTile(
+              value: alsoDelete,
+              onChanged: (value) =>
+                  setDialogState(() => alsoDelete = value ?? false),
+              contentPadding: EdgeInsets.zero,
+              controlAffinity: ListTileControlAffinity.leading,
+              title: const Text('Permanently delete this device'),
+            ),
+            if (alsoDelete) const _PermanentDeletionWarning(),
+          ],
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(context).pop(false),
+            child: const Text('Cancel'),
+          ),
+          TextButton(
+            style: TextButton.styleFrom(foregroundColor: colorScheme.error),
+            onPressed: () => Navigator.of(context).pop(true),
+            child: Text(alsoDelete ? 'Delete' : 'Forget'),
+          ),
+        ],
+      ),
+    ),
+  );
+
+  if (action != true || !context.mounted) return;
+
+  try {
+    if (alsoDelete) {
+      await BackendAPI.instance.deleteDevice(device.macAddress);
+      if (!context.mounted) return;
+      UISnackbars.showSuccess(context, 'Device deleted');
+      (onDeleted ?? onRefresh)();
+    } else {
+      await BackendAPI.instance.forgetDevice(device.macAddress);
+      if (!context.mounted) return;
+      UISnackbars.showSuccess(context, 'Device forgotten');
+      onRefresh();
+    }
+  } catch (e) {
+    if (!context.mounted) return;
+    UISnackbars.showError(
+      context,
+      alsoDelete
+          ? 'Failed to delete device. ${dioErrorToUserMessage(e)}'
+          : 'Failed to forget device. ${dioErrorToUserMessage(e)}',
+    );
+  }
+}
+
+/// Confirms and performs the permanent deletion of a not-registered device,
+/// erasing the device record and all of its event history.
+///
+/// On success [onDeleted] is invoked when provided (e.g. to navigate away from
+/// a now-stale detail page); otherwise [onRefresh] is used.
+Future<void> confirmDeleteDevice(
+  BuildContext context,
+  Device device,
+  VoidCallback onRefresh, {
+  VoidCallback? onDeleted,
+}) async {
   final colorScheme = Theme.of(context).colorScheme;
 
   final confirmed = await showDialog<bool>(
     context: context,
     builder: (context) => AlertDialog(
-      title: const Text('Forget Device'),
-      content: Text(
-        'This device will be unregistered and will no longer be linked to '
-        '${device.owner}. Are you sure?',
+      title: const Text('Delete Device'),
+      content: const Column(
+        mainAxisSize: MainAxisSize.min,
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Text('Are you sure you want to delete this device?'),
+          SizedBox(height: Insets.md),
+          _PermanentDeletionWarning(),
+        ],
       ),
       actions: [
         TextButton(
@@ -27,8 +118,9 @@ Future<void> confirmForgetDevice(
           child: const Text('Cancel'),
         ),
         TextButton(
+          style: TextButton.styleFrom(foregroundColor: colorScheme.error),
           onPressed: () => Navigator.of(context).pop(true),
-          child: Text('Forget', style: TextStyle(color: colorScheme.error)),
+          child: const Text('Delete'),
         ),
       ],
     ),
@@ -37,15 +129,46 @@ Future<void> confirmForgetDevice(
   if (confirmed != true || !context.mounted) return;
 
   try {
-    await BackendAPI.instance.forgetDevice(device.macAddress);
+    await BackendAPI.instance.deleteDevice(device.macAddress);
     if (!context.mounted) return;
-    UISnackbars.showSuccess(context, 'Device forgotten');
-    onRefresh();
+    UISnackbars.showSuccess(context, 'Device deleted');
+    (onDeleted ?? onRefresh)();
   } catch (e) {
     if (!context.mounted) return;
     UISnackbars.showError(
       context,
-      'Failed to forget device. ${dioErrorToUserMessage(e)}',
+      'Failed to delete device. ${dioErrorToUserMessage(e)}',
+    );
+  }
+}
+
+/// Embedded warning shown when a permanent deletion is about to happen.
+class _PermanentDeletionWarning extends StatelessWidget {
+  const _PermanentDeletionWarning();
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
+    return Container(
+      padding: const EdgeInsets.all(Insets.md),
+      decoration: BoxDecoration(
+        color: colorScheme.errorContainer,
+        borderRadius: BorderRadius.circular(Insets.sm),
+      ),
+      child: Row(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Icon(Icons.warning_amber, color: colorScheme.onErrorContainer),
+          const SizedBox(width: Insets.sm),
+          Expanded(
+            child: Text(
+              'This permanently erases the device and its entire event '
+              'history. This action cannot be undone.',
+              style: TextStyle(color: colorScheme.onErrorContainer),
+            ),
+          ),
+        ],
+      ),
     );
   }
 }
@@ -136,7 +259,7 @@ Future<void> showEditDeviceDialog(
               onPressed: () => Navigator.of(context).pop(false),
               child: const Text('Cancel'),
             ),
-            TextButton(onPressed: save, child: const Text('Save')),
+            FilledButton(onPressed: save, child: const Text('Save')),
           ],
         );
       },
@@ -240,7 +363,7 @@ Future<void> showRegisterDeviceDialog(
               onPressed: () => Navigator.of(context).pop(false),
               child: const Text('Cancel'),
             ),
-            TextButton(onPressed: save, child: const Text('Save')),
+            FilledButton(onPressed: save, child: const Text('Register')),
           ],
         );
       },