Replace the basic <select> dropdown in the route encounter form (AdminRouteDetail) with the PokemonSelector autocomplete component already used in the boss team editor.
## Current behavior
- Route encounter form has a plain HTML select dropdown listing all pokemon
- Difficult to find a specific pokemon in a list of 1000+
## Desired behavior
- Use the existing PokemonSelector component (autocomplete with search, sprite preview, dex number)
- Same UX as the boss team editor pokemon selection
## Files
- frontend/src/pages/admin/AdminRouteDetail.tsx — update the form
- frontend/src/components/admin/RouteEncounterFormModal.tsx — replace select with PokemonSelector
