Fix TypeScript errors in frontend build

Cast boss type select value to union type and remove unused
AdvanceLegInput import.

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

frontend/src/components/admin/BossBattleFormModal.tsx

@@ -107,7 +107,7 @@ export function BossBattleFormModal({
           <label className="block text-sm font-medium mb-1">Type</label>
           <select
             value={bossType}
-            onChange={(e) => setBossType(e.target.value)}
+            onChange={(e) => setBossType(e.target.value as typeof bossType)}
             className="w-full px-3 py-2 border rounded-md dark:bg-gray-700 dark:border-gray-600"
           >
             {BOSS_TYPES.map((t) => (