Reduce navigation depth when working with route encounters. Currently requires: Games > Game > Routes tab > Click route, and there's no way to jump between sibling routes.
## Desired behavior
- On the route detail page (AdminRouteDetail), add prev/next route navigation buttons to quickly move between routes in the same game
- Consider showing a sidebar or dropdown with all routes for quick jumping
- Breadcrumbs should be clickable to go back to the game detail
## Notes
- The route order is already available — sort by order to determine prev/next
- This is particularly useful when populating encounter data for a new game, going route by route
