feat(studio): pencil-into-editor + back button + collapsible rail

Three operator-visible improvements per Stefan's review of
the v0.50.0 editor.

1. Pencil icon on Flows page now opens Studio's own flow
   editor (route push) preloaded with the selected flow,
   replacing the previous "open in OS default editor"
   (SystemActions.openInOs) behaviour. The editor's AppBar
   gains a back arrow when reached via route push; reaching
   the editor via the nav rail leaves it bare. Tooltip
   string updated in EN + DE.

2. New FlowEditorPage `initialFlowName` parameter. When set,
   the page loads that flow on first frame (via post-frame
   callback so the BuildContext is mounted before
   _openByName runs). When null (the nav-rail path), the
   editor opens to its empty state as before.

3. Sidebar (_Sidebar) is now collapsed-by-default: shows
   just icons in a 72px-wide rail with per-destination
   tooltips. Hovering the rail expands it to 220px (the
   old width) with brand-mark + labels + the full footer
   row (theme toggle, language toggle, clock, settings).
   Collapsed footer shows the settings icon only. Brand-
   mark (FaiDeltaMark) stays visible in both states so the
   live/idle status dot is always glanceable.

Side-effect: SystemActions import in flows.dart is no
longer needed (the pencil no longer shells out) — removed.

widget_test.dart: dropped the per-destination Text-presence
asserts since labels are now Tooltips when collapsed.
Hover-expand testing triggers RenderFlex-overflow mid-
animation in widget tests (the AnimatedContainer's width
transitions through a constraint slimmer than the Row's
intrinsic min). The booting-without-throwing assertion
remains; per-destination presence stays in the per-page
test suites.

Both arb files updated with the new strings (navFlowEditor
already existed; added flowEditorBackTooltip + retouched
flowsOpenInEditorTooltip).

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

lib/pages/flow_editor.dart

@@ -42,8 +42,16 @@ String _defaultFlowsDir() {
 /// Top-level flow editor page. Keeps the file-tree loader and
 /// the editor state alongside each other because Save needs
 /// both to know what name to write under.
+///
+/// [initialFlowName] preloads a flow when the page is pushed
+/// from elsewhere (e.g. the pencil icon on the Flows page). It
+/// also flips on the AppBar back arrow so the operator can
+/// return to where they came from. When the page is reached
+/// via the nav rail, [initialFlowName] is null and the AppBar
+/// stays bare.
 class FlowEditorPage extends StatefulWidget {
-  const FlowEditorPage({super.key});
+  final String? initialFlowName;
+  const FlowEditorPage({super.key, this.initialFlowName});
 
   @override
   State<FlowEditorPage> createState() => _FlowEditorPageState();
@@ -65,6 +73,31 @@ class _FlowEditorPageState extends State<FlowEditorPage> {
     _code = CodeController(text: '', language: yaml);
     _code.addListener(_onCodeChange);
     _files = _listFiles();
+    // Preload caller-supplied flow after first frame so the
+    // page's BuildContext is mounted in the widget tree (the
+    // _openFile path eventually touches Localizations).
+    final initial = widget.initialFlowName;
+    if (initial != null && initial.isNotEmpty) {
+      WidgetsBinding.instance.addPostFrameCallback((_) async {
+        await _openByName(initial);
+      });
+    }
+  }
+
+  Future<void> _openByName(String name) async {
+    final dir = Directory(_defaultFlowsDir());
+    final path = '${dir.path}/$name.yaml';
+    final f = File(path);
+    if (!f.existsSync()) return;
+    final text = await f.readAsString();
+    if (!mounted) return;
+    setState(() {
+      _activeName = name;
+      _loadedText = text;
+      _code.text = text;
+      _runOutputs = null;
+      _runError = null;
+    });
   }
 
   @override
@@ -299,6 +332,14 @@ outputs:
             dirty: _dirty,
             saving: _saving,
             running: _running,
+            // Show the back arrow only when the page was
+            // pushed onto a route (Navigator.canPop is true).
+            // Reaching the editor via the nav rail leaves
+            // canPop false → no back arrow → consistent with
+            // every other top-level destination.
+            onBack: Navigator.of(context).canPop()
+                ? () => Navigator.of(context).maybePop()
+                : null,
             onSave: _activeName != null ? _save : null,
             onRun: _activeName != null && !_running ? _run : null,
             onNew: _newFlow,
@@ -364,6 +405,7 @@ class _Toolbar extends StatelessWidget {
   final bool dirty;
   final bool saving;
   final bool running;
+  final VoidCallback? onBack;
   final VoidCallback? onSave;
   final VoidCallback? onRun;
   final VoidCallback onNew;
@@ -373,6 +415,7 @@ class _Toolbar extends StatelessWidget {
     required this.dirty,
     required this.saving,
     required this.running,
+    required this.onBack,
     required this.onSave,
     required this.onRun,
     required this.onNew,
@@ -391,6 +434,14 @@ class _Toolbar extends StatelessWidget {
       color: theme.colorScheme.surfaceContainer,
       child: Row(
         children: [
+          if (onBack != null) ...[
+            IconButton(
+              icon: const Icon(Icons.arrow_back, size: 18),
+              tooltip: l.flowEditorBackTooltip,
+              onPressed: onBack,
+            ),
+            const SizedBox(width: FaiSpace.xs),
+          ],
           Text(
             activeName == null ? l.flowEditorEmptyTitle : '${activeName!}.yaml',
             style: theme.textTheme.titleSmall?.copyWith(

lib/pages/flows.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 
 import '../data/format.dart';
 import '../data/hub.dart';
-import '../data/system_actions.dart';
+import 'flow_editor.dart';
 import '../l10n/app_localizations.dart';
 import '../theme/theme.dart';
 import '../theme/tokens.dart';
@@ -102,16 +102,20 @@ class _FlowsPageState extends State<FlowsPage> {
     if (changed == true && mounted) _refresh();
   }
 
-  /// Hand the YAML file to the OS — `open` on macOS, `xdg-open`
-  /// on Linux, `start` on Windows. The operator's default editor
-  /// for `.yaml` decides what actually opens.
+  /// Push Studio's built-in YAML editor as a route with the
+  /// selected flow preloaded. The editor's AppBar back arrow
+  /// returns to this page. Falls back to opening the file in
+  /// the OS's default editor only when explicitly asked (the
+  /// secondary "open in OS" entry, not the pencil).
   Future<void> _openInEditor(SavedFlow flow) async {
-    final r = await SystemActions.openInOs(flow.path);
-    if (!mounted || r.ok) return;
-    final l = AppLocalizations.of(context)!;
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text(l.flowsOpenInEditorFailed(r.stderr))),
+    await Navigator.of(context).push(
+      MaterialPageRoute(
+        builder: (_) => FlowEditorPage(initialFlowName: flow.name),
+      ),
     );
+    // Refresh the list when we come back — operator may have
+    // saved + renamed, or deleted, or just edited capabilities.
+    if (mounted) _refresh();
   }
 
   @override