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>

pubspec.yaml

@@ -0,0 +1,23 @@
+name: fai_studio_flow_editor
+description: Swappable inline YAML editor for F∆I Studio flows.
+version: 0.1.0
+publish_to: 'none'
+repository: https://git.flemming.ai/fai/studio-flow-editor
+
+environment:
+  sdk: ^3.11.0-200.1.beta
+  flutter: '>=3.30.0'
+
+dependencies:
+  flutter:
+    sdk: flutter
+  # CodeMirror-style code editor + the highlight package's
+  # YAML grammar. Both are pure Dart so the editor stays
+  # WebView-free and ships in the same binary.
+  flutter_code_editor: ^0.3.5
+  highlight: ^0.7.0
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^6.0.0