feat(08-02): add delete button and confirmation dialog to TaskFormScreen

- Red FilledButton.icon with error color below history section (edit mode only)
- _onDelete shows AlertDialog with taskDeleteConfirmTitle/Message l10n strings
- Confirm calls smartDeleteTask and pops back to room task list
- Cancel dismisses dialog with no action
- Button disabled while _isLoading
- All 144 tests pass, dart analyze clean

lib/features/tasks/presentation/task_form_screen.dart

@@ -201,6 +201,21 @@ class _TaskFormScreenState extends ConsumerState<TaskFormScreen> {
                   taskId: widget.taskId!,
                 ),
               ),
+              // DELETE BUTTON
+              const Divider(),
+              const SizedBox(height: 8),
+              SizedBox(
+                width: double.infinity,
+                child: FilledButton.icon(
+                  style: FilledButton.styleFrom(
+                    backgroundColor: theme.colorScheme.error,
+                    foregroundColor: theme.colorScheme.onError,
+                  ),
+                  onPressed: _isLoading ? null : _onDelete,
+                  icon: const Icon(Icons.delete_outline),
+                  label: Text(l10n.taskDeleteConfirmAction),
+                ),
+              ),
             ],
           ],
         ),
@@ -452,6 +467,44 @@ class _TaskFormScreenState extends ConsumerState<TaskFormScreen> {
       }
     }
   }
+
+  Future<void> _onDelete() async {
+    final l10n = AppLocalizations.of(context);
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: Text(l10n.taskDeleteConfirmTitle),
+        content: Text(l10n.taskDeleteConfirmMessage),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, false),
+            child: Text(l10n.cancel),
+          ),
+          FilledButton(
+            style: FilledButton.styleFrom(
+              backgroundColor: Theme.of(ctx).colorScheme.error,
+            ),
+            onPressed: () => Navigator.pop(ctx, true),
+            child: Text(l10n.taskDeleteConfirmAction),
+          ),
+        ],
+      ),
+    );
+
+    if (confirmed != true || !mounted) return;
+
+    setState(() => _isLoading = true);
+    try {
+      await ref.read(taskActionsProvider.notifier).smartDeleteTask(widget.taskId!);
+      if (mounted) {
+        context.pop();
+      }
+    } finally {
+      if (mounted) {
+        setState(() => _isLoading = false);
+      }
+    }
+  }
 }
 
 /// Unit options for custom frequency input.