feat(studio): inline flow editor — YAML highlighting + Run (v0.50.0)

New top-level destination "Editor" (Cmd+5) ships as Studio's
fifth surface. The editor reads + writes flow YAML directly
under ~/.fai/data/flows/ via dart:io — the hub picks up the
changes on the next listFlows / runSavedFlow call.

Layout: two-pane shell. Left (240 px) is the file list; right
flexes to the code pane and an optional results column when
Run produces output. Top toolbar exposes:
  * filename + dirty-mark
  * New flow (scaffolds from a debug.echo template)
  * Save (writes the active file to disk)
  * Run (saves first if dirty, calls
    HubService.runSavedFlow, surfaces typed FlowOutputs in
    a side panel)
  * Refresh

YAML highlighting via flutter_code_editor + the highlight
package's yaml language. Lightweight style map mapping the
five token classes that actually appear in flow YAML
(attr / string / number / comment / subst for the
${{ ... }} template syntax) to FaiTheme colors — keeps the
editor visually consistent with the rest of Studio.

New-flow naming uses a FilteringTextInputFormatter that
restricts the name to [a-z0-9_-]. A "name already exists"
SnackBar surfaces the conflict instead of silently
overwriting.

Bilingual strings shipped (en.arb + de.arb) for every
operator-facing string: toolbar buttons, dialogs, empty
states, file-exists error, run-output header.

New deps:
  * flutter_code_editor ^0.3.5
  * highlight (transitive — pinned as direct so the
    yaml-language import has its declared dependency).

Smoke-test (test/flow_editor_test.dart) pumps the page and
asserts the empty-state + toolbar render without throwing
on hosts that don't have ~/.fai/data/flows yet. The full
file-list + open-on-tap flow needs a writable HOME override
which dart:io's read-only Platform.environment doesn't allow
inside a test isolate — that path lives in the integration
suite as a follow-up.

Version bumps:
  * pubspec.yaml: 0.49.1 → 0.50.0
  * main.dart kStudioVersion: 0.42.0 → 0.50.0 (had drifted
    behind pubspec; brought back into sync as part of this
    bump)

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/main.dart

@@ -16,6 +16,7 @@ import 'l10n/app_localizations.dart';
 import 'pages/approvals.dart';
 import 'pages/audit.dart';
 import 'pages/doctor.dart';
+import 'pages/flow_editor.dart';
 import 'pages/flows.dart';
 import 'pages/store.dart';
 import 'pages/welcome.dart';
@@ -27,7 +28,7 @@ import 'widgets/widgets.dart';
 /// Studio's own build version. Bump on every UI commit so the
 /// running app self-identifies — visible in the sidebar header
 /// and quick-glance proof that you're seeing the current build.
-const String kStudioVersion = '0.42.0';
+const String kStudioVersion = '0.50.0';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
@@ -234,6 +235,12 @@ class _StudioShellState extends State<StudioShell> {
       selectedIcon: Icons.account_tree,
       page: FlowsPage(),
     ),
+    _NavPage(
+      id: 'flow-editor',
+      icon: Icons.code_outlined,
+      selectedIcon: Icons.code,
+      page: FlowEditorPage(),
+    ),
     _NavPage(
       id: 'audit',
       icon: Icons.timeline_outlined,
@@ -834,6 +841,8 @@ class _NavPage {
         return l.navStore;
       case 'flows':
         return l.navFlows;
+      case 'flow-editor':
+        return l.navFlowEditor;
       case 'audit':
         return l.navAudit;
       case 'approvals':