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>

.gitignore

@@ -0,0 +1,12 @@
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+build/
+.idea/
+.vscode/
+*.iml
+.DS_Store
+pubspec.lock