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/utils/edgeColors.ts

@@ -0,0 +1,9 @@
+import type { EdgeType } from '@/types'
+
+export const EDGE_DEFAULT_COLORS: Record<EdgeType, string> = {
+  ethernet: '#30363d',
+  wifi: '#00d4ff',
+  iot: '#e3b341',
+  vlan: '#00d4ff',
+  virtual: '#8b949e',
+}