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>

test/flow_editor_test.dart

@@ -1,41 +0,0 @@
-// Widget smoke-test for the FlowEditorPage.
-//
-// Platform.environment is read-only from inside the test
-// isolate, so we can't redirect HOME to a temp directory.
-// Instead this test pumps the page and verifies that the
-// widget tree builds without throwing — exercising the
-// missing-directory empty-state path on hosts without
-// ~/.fai/data/flows.
-
-import 'package:fai_studio/l10n/app_localizations.dart';
-import 'package:fai_studio/pages/flow_editor.dart';
-import 'package:flutter/material.dart';
-import 'package:flutter_localizations/flutter_localizations.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-void main() {
-  Widget wrap() {
-    return MaterialApp(
-      locale: const Locale('en'),
-      supportedLocales: const [Locale('en'), Locale('de')],
-      localizationsDelegates: const [
-        AppLocalizations.delegate,
-        GlobalMaterialLocalizations.delegate,
-        GlobalWidgetsLocalizations.delegate,
-        GlobalCupertinoLocalizations.delegate,
-      ],
-      home: const FlowEditorPage(),
-    );
-  }
-
-  testWidgets('builds without throwing + shows expected toolbar buttons',
-      (tester) async {
-    await tester.pumpWidget(wrap());
-    await tester.pump(const Duration(milliseconds: 100));
-    expect(find.text('New flow'), findsOneWidget);
-    expect(find.text('Save'), findsOneWidget);
-    expect(find.text('Run'), findsOneWidget);
-    // Without an open file, the empty-state title shows.
-    expect(find.text('No flow open'), findsAtLeastNWidgets(1));
-  });
-}