feat: rename Rectangle to Zone, add border width selector

- Rename "Rectangle" → "Zone" in sidebar, add modal and edit modal
- Add border width selector (1–5px, default 2px) to the Zone modal
- Border width persisted in custom_colors.border_width and applied in GroupRectNode

frontend/src/App.tsx

@@ -182,6 +182,7 @@ export default function App() {
         custom_colors: {
           border: data.border_color,
           border_style: data.border_style,
+          border_width: data.border_width,
           background: data.background_color,
           text_color: data.text_color,
           text_position: data.text_position,
@@ -198,6 +199,7 @@ export default function App() {
 
   const handleUpdateGroupRect = useCallback((data: GroupRectFormData) => {
     if (!editingGroupRectId) return
+    snapshotHistory()
     const existing = nodes.find((n) => n.id === editingGroupRectId)
     updateNode(editingGroupRectId, {
       label: data.label,
@@ -205,6 +207,7 @@ export default function App() {
         ...existing?.data.custom_colors,
         border: data.border_color,
         border_style: data.border_style,
+        border_width: data.border_width,
         background: data.background_color,
         text_color: data.text_color,
         text_position: data.text_position,
@@ -214,7 +217,7 @@ export default function App() {
     })
     setNodeZIndex(editingGroupRectId, data.z_order - 10)
     setEditingGroupRectId(null)
-  }, [editingGroupRectId, nodes, updateNode, setNodeZIndex, setEditingGroupRectId])
+  }, [editingGroupRectId, nodes, updateNode, setNodeZIndex, setEditingGroupRectId, snapshotHistory])
 
   const handleDeleteGroupRect = useCallback(() => {
     if (!editingGroupRectId) return
@@ -436,7 +439,7 @@ export default function App() {
           open={addGroupRectOpen}
           onClose={() => setAddGroupRectOpen(false)}
           onSubmit={handleAddGroupRect}
-          title="Add Rectangle"
+          title="Add Zone"
         />
 
         {/* key forces re-mount when editing a different rect */}
@@ -457,11 +460,12 @@ export default function App() {
               text_position: rc.text_position ?? 'top-left',
               border_color: rc.border ?? '#00d4ff',
               border_style: rc.border_style ?? 'solid',
+              border_width: rc.border_width ?? 2,
               background_color: rc.background ?? '#00d4ff0d',
               z_order: rc.z_order ?? 1,
             }
           })()}
-          title="Edit Rectangle"
+          title="Edit Zone"
         />
 
         {/* key forces re-mount on open so useState captures current theme as original */}