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

frontend/src/types/index.ts

@@ -49,6 +49,7 @@ export interface EdgeData extends Record<string, unknown> {
   label?: string
   vlan_id?: number
   speed?: string
+  custom_color?: string
 }
 
 export const NODE_TYPE_LABELS: Record<NodeType, string> = {