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>

.metadata

@@ -0,0 +1,36 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "ebde138e3886dc1a68d4dfc4e84cc02d712eef69"
+  channel: "beta"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+      base_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+    - platform: linux
+      create_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+      base_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+    - platform: macos
+      create_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+      base_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+    - platform: windows
+      create_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+      base_revision: ebde138e3886dc1a68d4dfc4e84cc02d712eef69
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'