feat: F∆I Studio MVP scaffold (Tier-2 desktop GUI)

Initial scaffold for the F∆I Platform Tier-2 generic GUI client.
Flutter Desktop (macOS, Linux, Windows). Three MVP pages with
mock data, sharing one navigation shell:

- Modules — installed modules with capabilities, declared
  permissions and required services.
- Audit — event-stream view with type filter and tone-coded
  rows (started / completed / failed).
- Approvals — pending system.approval@^0 reviews with prompt,
  payload preview, and approve/reject buttons.

Live gRPC connection arrives in the next iteration via
fai_dart_sdk (sibling repo, currently a typed stub).

Future Forgejo path: fai/studio. Local layout matches existing
fai_platform/ convention.

Background: see docs/architecture/client.md in the platform
repo. The tier-2 client was previously called "Stage" — renamed
to "Studio" on 2026-05-05 to avoid confusion with
"staging environment".

flutter analyze: clean. flutter test: 2/2.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/widget_test.dart

@@ -0,0 +1,28 @@
+// Smoke test: app boots, navigation rail shows the three MVP
+// pages, and the Modules page renders the mocked module list.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fai_studio/main.dart';
+
+void main() {
+  testWidgets('Studio shell shows the three MVP destinations',
+      (tester) async {
+    await tester.pumpWidget(const StudioApp());
+    await tester.pump();
+
+    expect(find.text('F∆I Studio'), findsOneWidget);
+    expect(find.text('Modules'), findsWidgets);
+    expect(find.text('Audit'), findsOneWidget);
+    expect(find.text('Approvals'), findsOneWidget);
+  });
+
+  testWidgets('Modules page lists the mocked modules',
+      (tester) async {
+    await tester.pumpWidget(const StudioApp());
+    await tester.pump();
+
+    expect(find.text('echo'), findsOneWidget);
+    expect(find.text('text-extract'), findsOneWidget);
+    expect(find.text('llm-chat'), findsOneWidget);
+  });
+}