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/data/mock.dart

@@ -0,0 +1,165 @@
+// Mock data for the MVP. Replaced by live gRPC calls via
+// fai_dart_sdk in the next iteration.
+
+class ModuleSummary {
+  final String name;
+  final String version;
+  final List<String> capabilities;
+  final List<String> permissions;
+  final List<String> requiresServices;
+
+  const ModuleSummary({
+    required this.name,
+    required this.version,
+    required this.capabilities,
+    required this.permissions,
+    required this.requiresServices,
+  });
+}
+
+class AuditEvent {
+  final String eventId;
+  final DateTime timestamp;
+  final String type;
+  final String? flowName;
+  final String? stepId;
+  final String? moduleName;
+  final int? durationMs;
+  final String? error;
+
+  const AuditEvent({
+    required this.eventId,
+    required this.timestamp,
+    required this.type,
+    this.flowName,
+    this.stepId,
+    this.moduleName,
+    this.durationMs,
+    this.error,
+  });
+}
+
+class PendingApproval {
+  final String id;
+  final String flowName;
+  final String stepId;
+  final String prompt;
+  final String? showPreview;
+  final DateTime createdAt;
+  final DateTime? expiresAt;
+
+  const PendingApproval({
+    required this.id,
+    required this.flowName,
+    required this.stepId,
+    required this.prompt,
+    this.showPreview,
+    required this.createdAt,
+    this.expiresAt,
+  });
+}
+
+const mockModules = <ModuleSummary>[
+  ModuleSummary(
+    name: 'echo',
+    version: '0.1.0',
+    capabilities: ['debug.echo@0.1.0'],
+    permissions: [],
+    requiresServices: [],
+  ),
+  ModuleSummary(
+    name: 'text-extract',
+    version: '0.1.1',
+    capabilities: ['text.extract@0.1.0'],
+    permissions: [],
+    requiresServices: [],
+  ),
+  ModuleSummary(
+    name: 'llm-chat',
+    version: '0.1.0',
+    capabilities: ['llm.chat@0.1.0'],
+    permissions: [
+      'net: localhost',
+      'net: 127.0.0.1',
+      'net: api.openai.com',
+      'net: api.anthropic.com',
+    ],
+    requiresServices: ['ollama'],
+  ),
+  ModuleSummary(
+    name: 'text-translate',
+    version: '0.1.0',
+    capabilities: ['text.translate@0.1.0'],
+    permissions: [
+      'net: localhost',
+      'net: 127.0.0.1',
+    ],
+    requiresServices: ['ollama'],
+  ),
+  ModuleSummary(
+    name: 'text-summarize',
+    version: '0.1.0',
+    capabilities: ['text.summarize@0.1.0'],
+    permissions: [
+      'net: localhost',
+      'net: 127.0.0.1',
+    ],
+    requiresServices: ['ollama'],
+  ),
+];
+
+final mockEvents = <AuditEvent>[
+  AuditEvent(
+    eventId: '01926f8a-7c34-7b2c-9e4f-a8d3c1f9e2b4',
+    timestamp: DateTime.now().subtract(const Duration(seconds: 2)),
+    type: 'flow.completed',
+    flowName: 'extract-translate-summarize',
+    durationMs: 8421,
+  ),
+  AuditEvent(
+    eventId: '01926f8a-7b00-7000-aaaa-bbbbcccc0042',
+    timestamp: DateTime.now().subtract(const Duration(seconds: 3)),
+    type: 'step.completed',
+    flowName: 'extract-translate-summarize',
+    stepId: 'summarize',
+    moduleName: 'text-summarize',
+    durationMs: 3204,
+  ),
+  AuditEvent(
+    eventId: '01926f8a-7a90-7000-aaaa-bbbbcccc0041',
+    timestamp: DateTime.now().subtract(const Duration(seconds: 9)),
+    type: 'step.completed',
+    flowName: 'extract-translate-summarize',
+    stepId: 'review',
+    moduleName: 'system.approval',
+    durationMs: 5102,
+  ),
+  AuditEvent(
+    eventId: '01926f8a-7a00-7000-aaaa-bbbbcccc0040',
+    timestamp: DateTime.now().subtract(const Duration(seconds: 14)),
+    type: 'step.completed',
+    flowName: 'extract-translate-summarize',
+    stepId: 'translate',
+    moduleName: 'text-translate',
+    durationMs: 4815,
+  ),
+  AuditEvent(
+    eventId: '01926f8a-7800-7000-aaaa-bbbbcccc003f',
+    timestamp: DateTime.now().subtract(const Duration(seconds: 19)),
+    type: 'flow.started',
+    flowName: 'extract-translate-summarize',
+  ),
+];
+
+final mockApprovals = <PendingApproval>[
+  PendingApproval(
+    id: '01926f8a-9000-7b2c-9e4f-aaaa00000001',
+    flowName: 'extract-translate-summarize',
+    stepId: 'review',
+    prompt: 'Review the German→English translation before summarisation.',
+    showPreview:
+        '{ "translated": "The quarterly report shows..." }',
+    createdAt: DateTime.now().subtract(const Duration(minutes: 2)),
+    expiresAt: DateTime.now().add(const Duration(minutes: 28)),
+  ),
+];