feat(editor): success snackbar after save

Brief 1.5 s green-check snackbar appears at the bottom of
the editor after a successful save. Confirmation matters
here because Cmd+S is the operator's most common keystroke
in any editor — without visible feedback, they'd need to
hunt for the dirty dot to verify the save landed.

The error path's existing snackbar (which has no duration
cap by design — operators must read errors) stays
unchanged.

Signed-off-by: flemming-it <sf@flemming.it>

lib/src/flow_editor_page.dart

@@ -192,6 +192,25 @@ class _FlowEditorPageState extends State<FlowEditorPage>
       _controller.markSaved();
       _files = _listFiles();
       setState(() {});
+      // Brief positive feedback. Confirmation matters more
+      // here than on most save buttons because the operator
+      // can also save via Cmd+S without watching the dirty
+      // dot — a tiny green snackbar tells them the
+      // keystroke landed.
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          duration: const Duration(milliseconds: 1500),
+          behavior: SnackBarBehavior.floating,
+          content: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              const Icon(Icons.check_circle_outline, size: 16),
+              const SizedBox(width: FaiSpace.sm),
+              Text('$name.yaml ${_l.save.toLowerCase()}'),
+            ],
+          ),
+        ),
+      );
     } catch (e) {
       if (!mounted) return;
       ScaffoldMessenger.of(