Add child route (sub-area) management to route detail page

Shows a sub-areas section below encounters with add/delete support
and links to navigate to child route details.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/admin.ts

@@ -19,6 +19,7 @@ export interface UpdateGameInput {
 export interface CreateRouteInput {
   name: string
   order: number
+  parentRouteId?: number | null
   pinwheelZone?: number | null
 }