feat(studio): swap built-in flow editor for fai_studio_flow_editor package (v0.51.0)

The flow editor was internal to Studio (lib/pages/flow_editor.dart).
Per Stefan's review feedback ("austauschbar wäre schöner"),
extract it into its own Forgejo repo so the host can swap
the implementation without touching Studio.

New repo: https://git.flemming.ai/fai/studio-flow-editor

Studio's pubspec.yaml now references the package by git URL:

  dependencies:
    fai_studio_flow_editor:
      git:
        url: https://git.flemming.ai/fai/studio-flow-editor
        ref: main

To swap the editor:
  1. Fork (or write a new) fai/studio-flow-editor.
  2. Keep the FlowEditorPage(initialFlowName, locale, onRun)
     constructor signature — the stable host contract.
  3. Point the pubspec at your fork.
  4. Rebuild Studio.

Adapter pattern: _FlowEditorAdapter in main.dart resolves the
package's runtime dependencies (locale via Localizations,
onRun via HubService) from the BuildContext, then constructs
the package's FlowEditorPage. Same pattern in flows.dart for
the pencil → editor route push, so a future operator-side
locale switch propagates correctly.

The package brings its own copies of FaiSpace tokens, minimal
FaiEmptyState/FaiErrorBox widgets, and an inline EN+DE l10n
table — accepting a small amount of visual drift in exchange
for true package independence. flutter_code_editor +
highlight move from Studio's pubspec to the package's.

Deleted:
  lib/pages/flow_editor.dart  → package's lib/src/flow_editor_page.dart
  test/flow_editor_test.dart  → package's test/ (next commit there)

Bumped:
  pubspec.yaml version 0.50.0 → 0.51.0
  main.dart kStudioVersion 0.50.0 → 0.51.0
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/pages/flows.dart

@@ -5,9 +5,10 @@ import 'package:fai_client_sdk/fai_client_sdk.dart' show FlowInputDef;
 import 'package:file_picker/file_picker.dart';
 import 'package:flutter/material.dart';
 
+import 'package:fai_studio_flow_editor/fai_studio_flow_editor.dart';
+
 import '../data/format.dart';
 import '../data/hub.dart';
-import 'flow_editor.dart';
 import '../l10n/app_localizations.dart';
 import '../theme/theme.dart';
 import '../theme/tokens.dart';
@@ -108,9 +109,17 @@ class _FlowsPageState extends State<FlowsPage> {
   /// the OS's default editor only when explicitly asked (the
   /// secondary "open in OS" entry, not the pencil).
   Future<void> _openInEditor(SavedFlow flow) async {
+    final lang = Localizations.localeOf(context).languageCode;
     await Navigator.of(context).push(
       MaterialPageRoute(
-        builder: (_) => FlowEditorPage(initialFlowName: flow.name),
+        builder: (_) => FlowEditorPage(
+          initialFlowName: flow.name,
+          locale:
+              lang == 'de' ? FlowEditorLocale.de : FlowEditorLocale.en,
+          onRun: (name) async =>
+              (await HubService.instance.runSavedFlow(name: name))
+                  .cast<String, Object>(),
+        ),
       ),
     );
     // Refresh the list when we come back — operator may have