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/Runner/Configs/Warnings.xcconfig

@@ -0,0 +1,13 @@
+WARNING_CFLAGS = -Wall -Wconditional-uninitialized -Wnullable-to-nonnull-conversion -Wmissing-method-return-type -Woverlength-strings
+GCC_WARN_UNDECLARED_SELECTOR = YES
+CLANG_UNDEFINED_BEHAVIOR_SANITIZER_NULLABILITY = YES
+CLANG_WARN_UNGUARDED_AVAILABILITY = YES_AGGRESSIVE
+CLANG_WARN__DUPLICATE_METHOD_MATCH = YES
+CLANG_WARN_PRAGMA_PACK = YES
+CLANG_WARN_STRICT_PROTOTYPES = YES
+CLANG_WARN_COMMA = YES
+GCC_WARN_STRICT_SELECTOR_MATCH = YES
+CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES
+CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF = YES
+GCC_WARN_SHADOW = YES
+CLANG_WARN_UNREACHABLE_CODE = YES