EVA-121: right-click context menus for canvas, nodes, and edges

[suzil]

Summary

• New ContextMenu.tsx component: portalled floating menu with action/submenu/separator items; dismisses on outside click, Escape, or scroll; clamps to viewport edges
• canvasStore gains deleteNode, duplicateNode, deleteEdge actions (all undoable via existing applyNodeChanges/applyEdgeChanges)
• CanvasContainer wires onPaneContextMenu (Add Node submenu for all 5 types), onNodeContextMenu (Duplicate / Delete), onEdgeContextMenu (Delete); all gated behind !isOperate
• EdgePanel gets a keyboard-accessible "Delete edge" footer button (Trash2 icon, focus-visible ring)

Test plan

• Right-click empty canvas → Add Node submenu → each type places node at cursor position
• Right-click a node → Duplicate creates offset copy with same config; Delete removes it (both undo with Cmd+Z)
• Right-click an edge → Delete removes it
• Click "Delete edge" button in EdgePanel → edge removed, panel closes
• All buttons reachable by keyboard (Tab, Enter/Space)
• Context menus do not appear in Operate mode

Closes EVA-121

Made with Cursor