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.lock

@@ -120,6 +120,15 @@ packages:
       relative: true
     source: path
     version: "0.17.1"
+  fai_studio_flow_editor:
+    dependency: "direct main"
+    description:
+      path: "."
+      ref: main
+      resolved-ref: "51f9a1d2b1a292a109457ae29dd4e530397fd1c2"
+      url: "https://git.flemming.ai/fai/studio-flow-editor"
+    source: git
+    version: "0.1.0"
   fake_async:
     dependency: transitive
     description:
@@ -166,7 +175,7 @@ packages:
     source: sdk
     version: "0.0.0"
   flutter_code_editor:
-    dependency: "direct main"
+    dependency: transitive
     description:
       name: flutter_code_editor
       sha256: "9af48ba8e3558b6ea4bb98b84c5eb1649702acf53e61a84d88383eeb79b239b0"
@@ -261,7 +270,7 @@ packages:
     source: hosted
     version: "5.1.0"
   highlight:
-    dependency: "direct main"
+    dependency: transitive
     description:
       name: highlight
       sha256: "5353a83ffe3e3eca7df0abfb72dcf3fa66cc56b953728e7113ad4ad88497cf21"