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/types/index.ts

@@ -87,6 +87,11 @@ export interface NodeData extends Record<string, unknown> {
 
 export type EdgePathStyle = 'bezier' | 'smooth'
 
+export interface Waypoint {
+  x: number
+  y: number
+}
+
 export interface EdgeData extends Record<string, unknown> {
   type: EdgeType
   label?: string
@@ -95,6 +100,7 @@ export interface EdgeData extends Record<string, unknown> {
   custom_color?: string
   path_style?: EdgePathStyle
   animated?: boolean | 'snake' | 'flow' | 'none'
+  waypoints?: Waypoint[]
 }
 
 export const NODE_TYPE_LABELS: Record<NodeType, string> = {