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/utils/canvasSerializer.ts

@@ -1,5 +1,5 @@
 import type { Node, Edge } from '@xyflow/react'
-import type { NodeData, EdgeData } from '@/types'
+import type { NodeData, EdgeData, Waypoint } from '@/types'
 import { normalizeHandle } from '@/utils/handleUtils'
 
 // ── Types ────────────────────────────────────────────────────────────────────
@@ -46,6 +46,7 @@ export interface ApiEdge {
   animated?: boolean | 'snake' | 'flow' | 'none'
   source_handle?: string | null
   target_handle?: string | null
+  waypoints?: Waypoint[] | null
 }
 
 // ── Serialization (RF node → API save payload) ───────────────────────────────
@@ -121,6 +122,7 @@ export function serializeEdge(e: Edge<EdgeData>): Record<string, unknown> {
     animated: e.data?.animated ?? false,
     source_handle: normalizeHandle(e.sourceHandle),
     target_handle: normalizeHandle(e.targetHandle),
+    waypoints: e.data?.waypoints?.length ? e.data.waypoints : null,
   }
 }