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>

.gitignore

@@ -0,0 +1,45 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.build/
+.buildlog/
+.history
+.svn/
+.swiftpm/
+migrate_working_dir/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins-dependencies
+.pub-cache/
+.pub/
+/build/
+/coverage/
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release