feat: localize all UI text to Korean

Translate all user-facing strings to Korean across 25 files:
- UI screens: title, nickname, lobby, class select, waiting, game,
  shop, result, help, leaderboard, achievements, codex, stats
- Game logic: combat logs, events, achievements, mutations, emotes,
  lobby errors, session messages
- Keep English for: class names, monster names, item names, relic names

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dungeon/room.go

@@ -16,7 +16,7 @@ const (
 )
 
 func (r RoomType) String() string {
-	return [...]string{"Combat", "Treasure", "Shop", "Event", "Empty", "Boss", "Secret", "MiniBoss"}[r]
+	return [...]string{"전투", "보물", "상점", "이벤트", "빈 방", "보스", "비밀", "미니보스"}[r]
 }
 
 type Tile int