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>

README.md

@@ -0,0 +1,50 @@
+# F∆I Studio
+
+Desktop GUI client for the F∆I Platform hub. Tier-2 generic
+platform client per `docs/architecture/client.md`. Connects to a
+local or remote `fai serve` over gRPC.
+
+> **Status (2026-05-05):** MVP scaffold. Three pages with mock
+> data, no live gRPC connection yet. Validates the visual
+> direction before the Dart SDK is wired in.
+
+## Pages (MVP scope)
+
+1. **Modules** — installed modules, capabilities, declared
+   permissions. Click → manifest detail.
+2. **Audit** — live stream of hub events (when wired). Filter
+   by event type and flow.
+3. **Approvals** — pending `system.approval@^0` reviews with
+   approve / reject buttons.
+
+These three cover the platform's central properties:
+**composition** (modules), **audit** (events),
+**human-in-the-loop** (approvals).
+
+## Stack
+
+- Flutter 3.40+ (Desktop: macOS, Linux, Windows)
+- gRPC client via [`fai_dart_sdk`](../fai_dart_sdk) (sibling
+  repo; pinned by relative path during development)
+- No external runtime dependencies; bundles its own Dart VM
+
+## Run locally
+
+```bash
+flutter run -d macos
+```
+
+## Repo placement
+
+Will be published as `fai/studio` on Forgejo (`git.flemming.ws`).
+The local directory `fai_studio/` follows the established
+`fai_platform/` layout convention.
+
+## Why "Studio" and not "Stage"
+
+"Stage" collided too easily with "staging environment" in
+developer English. "Studio" is the established industry pattern
+for creator-tools (Visual Studio, Android Studio, RStudio) and
+matches the GUI's audience of module developers, operators, and
+power users. The rename was applied across all `docs/` in the
+platform repo on 2026-05-05.