feat: customizable link color per edge

- Add custom_color field to EdgeData type, Edge DB model and all schemas
- HomelableEdge applies custom_color as stroke override (before selected highlight)
- EDGE_DEFAULT_COLORS extracted to utils/edgeColors.ts (react-refresh compliant)
- EdgeModal: color picker row showing effective color (custom or type default)
  with hex value, Reset button when custom color is active
- Auto-migration adds custom_color column to edges table

backend/app/schemas/canvas.py

@@ -34,6 +34,7 @@ class EdgeSave(BaseModel):
     label: str | None = None
     vlan_id: int | None = None
     speed: str | None = None
+    custom_color: str | None = None
 
 
 class CanvasSaveRequest(BaseModel):