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>

macos/Flutter/Flutter-Debug.xcconfig

@@ -0,0 +1 @@
+#include "ephemeral/Flutter-Generated.xcconfig"

macos/Flutter/Flutter-Release.xcconfig

@@ -0,0 +1 @@
+#include "ephemeral/Flutter-Generated.xcconfig"

macos/Flutter/GeneratedPluginRegistrant.swift

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+//
+//  Generated file. Do not edit.
+//
+
+import FlutterMacOS
+import Foundation
+
+
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+}