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>

test/flow_editor_test.dart

@@ -0,0 +1,41 @@
+// 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));
+  });
+}