Add tabbed UI for routes/bosses and boss export endpoint

Refactors AdminGameDetail to use tabs instead of stacked sections,
adds GET /export/games/{game_id}/bosses endpoint, and adds Export
button to the Boss Battles tab.

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

frontend/src/api/admin.ts

@@ -94,6 +94,9 @@ export const exportGames = () =>
 export const exportGameRoutes = (gameId: number) =>
   api.get<{ filename: string; data: unknown }>(`/export/games/${gameId}/routes`)
 
+export const exportGameBosses = (gameId: number) =>
+  api.get<{ filename: string; data: unknown }>(`/export/games/${gameId}/bosses`)
+
 export const exportPokemon = () =>
   api.get<Record<string, unknown>[]>('/export/pokemon')