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/l10n/app_localizations.dart

@@ -3727,6 +3727,132 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Temporal'**
   String get storeSourceTemporal;
+
+  /// No description provided for @navFlowEditor.
+  ///
+  /// In en, this message translates to:
+  /// **'Editor'**
+  String get navFlowEditor;
+
+  /// No description provided for @flowEditorNew.
+  ///
+  /// In en, this message translates to:
+  /// **'New flow'**
+  String get flowEditorNew;
+
+  /// No description provided for @flowEditorSave.
+  ///
+  /// In en, this message translates to:
+  /// **'Save'**
+  String get flowEditorSave;
+
+  /// No description provided for @flowEditorRun.
+  ///
+  /// In en, this message translates to:
+  /// **'Run'**
+  String get flowEditorRun;
+
+  /// No description provided for @flowEditorRefresh.
+  ///
+  /// In en, this message translates to:
+  /// **'Refresh file list'**
+  String get flowEditorRefresh;
+
+  /// No description provided for @flowEditorRunOutput.
+  ///
+  /// In en, this message translates to:
+  /// **'RUN OUTPUT'**
+  String get flowEditorRunOutput;
+
+  /// No description provided for @flowEditorEmptyTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'No flow open'**
+  String get flowEditorEmptyTitle;
+
+  /// No description provided for @flowEditorEmptyBody.
+  ///
+  /// In en, this message translates to:
+  /// **'Pick one from the left, or click New flow to start a fresh one.'**
+  String get flowEditorEmptyBody;
+
+  /// No description provided for @flowEditorListEmptyTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'No saved flows'**
+  String get flowEditorListEmptyTitle;
+
+  /// No description provided for @flowEditorListEmptyBody.
+  ///
+  /// In en, this message translates to:
+  /// **'Click New flow to scaffold one from a template.'**
+  String get flowEditorListEmptyBody;
+
+  /// No description provided for @flowEditorDiscardTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Discard unsaved changes?'**
+  String get flowEditorDiscardTitle;
+
+  /// No description provided for @flowEditorDiscardBody.
+  ///
+  /// In en, this message translates to:
+  /// **'Switching files will throw away the edits you haven\'t saved.'**
+  String get flowEditorDiscardBody;
+
+  /// No description provided for @flowEditorDiscardKeep.
+  ///
+  /// In en, this message translates to:
+  /// **'Keep editing'**
+  String get flowEditorDiscardKeep;
+
+  /// No description provided for @flowEditorDiscardThrow.
+  ///
+  /// In en, this message translates to:
+  /// **'Throw away'**
+  String get flowEditorDiscardThrow;
+
+  /// No description provided for @flowEditorNewDialogTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'New flow'**
+  String get flowEditorNewDialogTitle;
+
+  /// No description provided for @flowEditorNewDialogLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Flow name'**
+  String get flowEditorNewDialogLabel;
+
+  /// No description provided for @flowEditorNewDialogHelper.
+  ///
+  /// In en, this message translates to:
+  /// **'lowercase letters, digits, hyphen, underscore'**
+  String get flowEditorNewDialogHelper;
+
+  /// No description provided for @flowEditorNewDialogCancel.
+  ///
+  /// In en, this message translates to:
+  /// **'Cancel'**
+  String get flowEditorNewDialogCancel;
+
+  /// No description provided for @flowEditorNewDialogCreate.
+  ///
+  /// In en, this message translates to:
+  /// **'Create'**
+  String get flowEditorNewDialogCreate;
+
+  /// No description provided for @flowEditorNewTemplateComment.
+  ///
+  /// In en, this message translates to:
+  /// **'Generated by Studio\'s Flow editor. Edit, save, run.'**
+  String flowEditorNewTemplateComment(String name);
+
+  /// No description provided for @flowEditorAlreadyExists.
+  ///
+  /// In en, this message translates to:
+  /// **'A flow named \"{name}\" already exists.'**
+  String flowEditorAlreadyExists(String name);
 }
 
 class _AppLocalizationsDelegate