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>

linux/runner/CMakeLists.txt

@@ -0,0 +1,26 @@
+cmake_minimum_required(VERSION 3.13)
+project(runner LANGUAGES CXX)
+
+# Define the application target. To change its name, change BINARY_NAME in the
+# top-level CMakeLists.txt, not the value here, or `flutter run` will no longer
+# work.
+#
+# Any new source files that you add to the application should be added here.
+add_executable(${BINARY_NAME}
+  "main.cc"
+  "my_application.cc"
+  "${FLUTTER_MANAGED_DIR}/generated_plugin_registrant.cc"
+)
+
+# Apply the standard set of build settings. This can be removed for applications
+# that need different build settings.
+apply_standard_settings(${BINARY_NAME})
+
+# Add preprocessor definitions for the application ID.
+add_definitions(-DAPPLICATION_ID="${APPLICATION_ID}")
+
+# Add dependency libraries. Add any application-specific dependencies here.
+target_link_libraries(${BINARY_NAME} PRIVATE flutter)
+target_link_libraries(${BINARY_NAME} PRIVATE PkgConfig::GTK)
+
+target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}")