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>

lib/main.dart

@@ -0,0 +1,137 @@
+// F∆I Studio — desktop GUI for the F∆I hub.
+//
+// MVP scope: three pages (Modules, Audit, Approvals) with mock
+// data, sharing one navigation shell. Live gRPC connection
+// arrives in the next iteration via fai_dart_sdk.
+
+import 'package:flutter/material.dart';
+
+import 'pages/approvals.dart';
+import 'pages/audit.dart';
+import 'pages/modules.dart';
+
+void main() {
+  runApp(const StudioApp());
+}
+
+class StudioApp extends StatelessWidget {
+  const StudioApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'F∆I Studio',
+      theme: ThemeData(
+        useMaterial3: true,
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: const Color(0xFF1E3A8A),
+          brightness: Brightness.light,
+        ),
+      ),
+      darkTheme: ThemeData(
+        useMaterial3: true,
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: const Color(0xFF1E3A8A),
+          brightness: Brightness.dark,
+        ),
+      ),
+      home: const StudioShell(),
+    );
+  }
+}
+
+class StudioShell extends StatefulWidget {
+  const StudioShell({super.key});
+
+  @override
+  State<StudioShell> createState() => _StudioShellState();
+}
+
+class _StudioShellState extends State<StudioShell> {
+  int _selectedIndex = 0;
+
+  static const _pages = <_NavPage>[
+    _NavPage(
+      label: 'Modules',
+      icon: Icons.extension_outlined,
+      selectedIcon: Icons.extension,
+      page: ModulesPage(),
+    ),
+    _NavPage(
+      label: 'Audit',
+      icon: Icons.timeline_outlined,
+      selectedIcon: Icons.timeline,
+      page: AuditPage(),
+    ),
+    _NavPage(
+      label: 'Approvals',
+      icon: Icons.inbox_outlined,
+      selectedIcon: Icons.inbox,
+      page: ApprovalsPage(),
+    ),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Scaffold(
+      body: Row(
+        children: [
+          NavigationRail(
+            extended: true,
+            minExtendedWidth: 180,
+            selectedIndex: _selectedIndex,
+            onDestinationSelected: (i) => setState(() => _selectedIndex = i),
+            leading: Padding(
+              padding: const EdgeInsets.symmetric(vertical: 16),
+              child: Column(
+                children: [
+                  Icon(Icons.hub_outlined, size: 32, color: theme.colorScheme.primary),
+                  const SizedBox(height: 8),
+                  Text(
+                    'F∆I Studio',
+                    style: theme.textTheme.titleMedium?.copyWith(
+                      color: theme.colorScheme.primary,
+                      fontWeight: FontWeight.w600,
+                    ),
+                  ),
+                  Text(
+                    'connected: localhost:50051',
+                    style: theme.textTheme.bodySmall?.copyWith(
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                  ),
+                ],
+              ),
+            ),
+            destinations: _pages
+                .map(
+                  (p) => NavigationRailDestination(
+                    icon: Icon(p.icon),
+                    selectedIcon: Icon(p.selectedIcon),
+                    label: Text(p.label),
+                  ),
+                )
+                .toList(),
+          ),
+          const VerticalDivider(width: 1),
+          Expanded(child: _pages[_selectedIndex].page),
+        ],
+      ),
+    );
+  }
+}
+
+class _NavPage {
+  final String label;
+  final IconData icon;
+  final IconData selectedIcon;
+  final Widget page;
+
+  const _NavPage({
+    required this.label,
+    required this.icon,
+    required this.selectedIcon,
+    required this.page,
+  });
+}