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/App.tsx

@@ -64,6 +64,7 @@ export default function App() {
         label: e.data?.label ?? null,
         vlan_id: e.data?.vlan_id ?? null,
         speed: e.data?.speed ?? null,
+        custom_color: e.data?.custom_color ?? null,
       }))
       await canvasApi.save({ nodes: nodesToSave, edges: edgesToSave, viewport: {} })
       markSaved()