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>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.50.0
+version: 0.51.0
 
 environment:
   sdk: ^3.11.0-200.1.beta
@@ -34,8 +34,14 @@ dependencies:
   # routes through the OS dialog so sandboxed Studio still
   # gets read access to the chosen file).
   file_picker: ^11.0.0
-  flutter_code_editor: ^0.3.5
-  highlight: ^0.7.0
+  # Swappable flow editor — lives in its own repo so it can
+  # be replaced without touching the rest of Studio. Pin to a
+  # commit (or branch) here; rebuild Studio to pick up
+  # editor-side changes.
+  fai_studio_flow_editor:
+    git:
+      url: https://git.flemming.ai/fai/studio-flow-editor
+      ref: main
 
 dev_dependencies:
   flutter_test: