feat(editor): right-click context menu + reset layout

Two operator-visible features filling concrete gaps the
jai_client editor exposed:

 - Right-click on any step node opens a popup menu at the
   cursor position with three actions:
   * Duplicate — clones the step with a unique id
     (<base>_2, <base>_3, …) and offsets its position so
     the copy is visible next to the original.
   * Disconnect all inputs — keeps every with-field key
     but clears its expression value, so the step still
     declares the same inputs but no longer wires them to
     any upstream step.
   * Delete — removes the step. Any dangling $refs in
     downstream steps become run-time errors; we don't
     silently rewrite their YAML because the surprise
     would be worse than the explicit error.

 - Floating "Reset layout" button on the canvas (next to
   Fit to screen). Wipes the layout sidecar so AutoLayout
   re-runs from scratch and the topological column
   placement is restored. Auto-fits afterwards. Useful
   when manual drags have drifted into spaghetti and the
   operator wants a clean slate without losing the flow.

Implementation:
 - FlowNode gains an `onContextMenu(Offset globalPos)`
   parameter, wired to GestureDetector.onSecondaryTapDown.
 - FlowEditorController.resetLayout() — clears layout,
   re-seeds via AutoLayout, saves sidecar, notifies.
 - The menu uses Flutter's showMenu() at the global
   cursor position for native-feeling positioning.

Version bumped 0.2.2 -> 0.3.0 — first new operator-visible
feature set since the v0.2 WYSIWYG release.

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

lib/src/widgets/flow_node.dart

@@ -67,6 +67,11 @@ class FlowNode extends StatelessWidget {
   final void Function(Offset delta)? onDrag;
   final VoidCallback? onDragEnd;
 
+  /// Right-click handler. Canvas wires this to a popup menu
+  /// at the cursor position (Duplicate / Delete / Disconnect
+  /// inputs etc.). `null` = no context menu offered.
+  final void Function(Offset globalPos)? onContextMenu;
+
   /// Live status from the most recent run, when this node is
   /// a step. Coloured dot in the header so the operator can
   /// glance at the canvas and see what's running.
@@ -84,6 +89,7 @@ class FlowNode extends StatelessWidget {
     this.onTap,
     this.onDrag,
     this.onDragEnd,
+    this.onContextMenu,
   });
 
   @override
@@ -99,6 +105,9 @@ class FlowNode extends StatelessWidget {
         onTap: onTap,
         onPanUpdate: onDrag == null ? null : (d) => onDrag!(d.delta),
         onPanEnd: onDragEnd == null ? null : (_) => onDragEnd!(),
+        onSecondaryTapDown: onContextMenu == null
+            ? null
+            : (details) => onContextMenu!(details.globalPosition),
         child: Material(
           color: theme.colorScheme.surfaceContainerHigh,
           elevation: selected ? 6 : 2,