feat: initial scaffold — swappable flow editor

Extracted from fai/studio's lib/pages/flow_editor.dart so the
editor can be swapped out independently of the host.

Public surface kept minimal — a single FlowEditorPage widget
with three named parameters (initialFlowName, locale, onRun).
The package brings its own design tokens, empty/error
widgets, l10n table; no host-internal types leak through.

Studio depends on this repo via pubspec.yaml git reference.
Forks point Studio at a different URL and rebuild.

See README.md for the swap recipe.

Signed-off-by: F∆I Platform <platform@flemming.ai>

README.md

@@ -0,0 +1,90 @@
+# fai_studio_flow_editor
+
+Swappable inline YAML editor for F∆I Studio flows.
+
+## Public surface
+
+```dart
+import 'package:fai_studio_flow_editor/fai_studio_flow_editor.dart';
+
+// Studio embeds the page as one of its destinations:
+FlowEditorPage(
+  initialFlowName: 'hello',                       // optional
+  locale: FlowEditorLocale.de,                    // or .en
+  onRun: (name) =>                                // optional callback
+      HubService.instance.runSavedFlow(name: name),
+)
+```
+
+- **`initialFlowName`** — when set, the page loads that flow
+  from `~/.fai/data/flows/<name>.yaml` on first build. Used
+  by Studio's Flows-page pencil → editor route push.
+- **`locale`** — picks EN or DE for the editor's internal
+  string table. Studio passes its active locale through.
+- **`onRun`** — async callback that runs the named flow
+  against the host hub and returns the typed outputs map.
+  When null, the Run button is disabled (e.g. when the
+  editor is embedded in a context without hub access).
+
+## Behaviour
+
+- File browser (left, 240 px) lists `*.yaml` in
+  `~/.fai/data/flows/`.
+- Code field (right) ships YAML syntax highlighting via
+  `flutter_code_editor` + the highlight package's yaml
+  grammar.
+- Save writes directly to disk via `dart:io`; the hub picks
+  up the change on its next `ListFlows` / `RunSavedFlow`
+  call.
+- Cmd+S / Ctrl+S — save. Cmd+Enter / Ctrl+Enter — run.
+
+## Swap it
+
+Studio depends on this package via `pubspec.yaml` git
+reference:
+
+```yaml
+dependencies:
+  fai_studio_flow_editor:
+    git:
+      url: https://git.flemming.ai/fai/studio-flow-editor
+      ref: main
+```
+
+To use a different editor:
+
+1. Fork this repo (or write your own from scratch).
+2. Keep the `FlowEditorPage` constructor signature
+   (`initialFlowName`, `locale`, `onRun`) — that's the
+   stable host contract.
+3. Point Studio's pubspec at your fork (or local-path
+   during development):
+   ```yaml
+   fai_studio_flow_editor:
+     git:
+       url: https://your-host/your/flow-editor
+       ref: main
+   ```
+4. Rebuild Studio. Done.
+
+The contract is intentionally minimal: a single page widget
+with three parameters. The package brings its own tokens,
+its own empty/error widgets, its own l10n table, so the
+host doesn't have to share internals. The trade-off is
+visual drift if Studio's design tokens change — that's the
+deal you accept for a swappable module.
+
+## Develop
+
+```bash
+flutter pub get
+flutter analyze
+flutter test
+```
+
+The package's own widget tests live in `test/`. End-to-end
+tests against a live hub stay in the host repo (Studio).
+
+## License
+
+Apache-2.0. Same as the rest of the F∆I platform.