feat: add interactive edge waypoints with smooth path editing

- Drag waypoints to reshape edges; double-click a waypoint to remove it
- + handles at segment midpoints to insert new waypoints
- Bezier style: catmull-rom smooth curves through waypoints
- Smooth style: rounded-corner polyline with soft 45° snap (snaps within 15px)
  - First + handle biased to source axis for perpendicular node exit
  - snap45both: ray-intersection solver ensures both adjacent segments snap to 45° simultaneously
- Clear path button in EdgeModal when waypoints exist
- Waypoints serialised/deserialised with canvas state

frontend/src/components/modals/EdgeModal.tsx

@@ -23,11 +23,12 @@ interface EdgeModalProps {
   onClose: () => void
   onSubmit: (data: EdgeData) => void
   onDelete?: () => void
+  onClearWaypoints?: () => void
   initial?: Partial<EdgeData>
   title?: string
 }
 
-export function EdgeModal({ open, onClose, onSubmit, onDelete, initial, title = 'Connect Nodes' }: EdgeModalProps) {
+export function EdgeModal({ open, onClose, onSubmit, onDelete, onClearWaypoints, initial, title = 'Connect Nodes' }: EdgeModalProps) {
   const [type, setType] = useState<EdgeType>(initial?.type ?? 'ethernet')
   const [label, setLabel] = useState(initial?.label ?? '')
   const [vlanId, setVlanId] = useState(initial?.vlan_id?.toString() ?? '')
@@ -175,6 +176,16 @@ export function EdgeModal({ open, onClose, onSubmit, onDelete, initial, title =
             </label>
           </div>
 
+          {onClearWaypoints && initial?.waypoints && initial.waypoints.length > 0 && (
+            <button
+              type="button"
+              onClick={() => { onClearWaypoints(); onClose() }}
+              className="text-[10px] text-muted-foreground hover:text-[#e3b341] transition-colors text-left"
+            >
+              Clear path ({initial.waypoints.length} point{initial.waypoints.length !== 1 ? 's' : ''})
+            </button>
+          )}
+
           <div className="flex justify-between gap-2 pt-1">
             {onDelete ? (
               <Button type="button" variant="ghost" size="sm" className="text-[#f85149] hover:text-[#f85149] hover:bg-[#f85149]/10" onClick={handleDelete}>