feat: 实现补杠和抢杠胡功能

新增 MeldType 和 MeldGroup 领域模型，支持碰、明杠、补杠、暗杠四种副露类型
GameSeat 新增副露管理方法，支持将碰升级为补杠
ResponseActionWindowBuilder 新增补杠响应窗口构建逻辑
SettlementService 新增补杠和抢杠胡结算规则
前端新增副露展示区域，支持显示各类副露标签

frontend/src/App.vue

@@ -35,6 +35,7 @@ type SelfSeatView = {
   score: number
   handTiles: string[]
   discardTiles: string[]
+  melds: string[]
 }
 
 type PublicSeatView = {
@@ -47,6 +48,7 @@ type PublicSeatView = {
   score: number
   handCount: number
   discardTiles: string[]
+  melds: string[]
 }
 
 type GameStateResponse = {
@@ -543,6 +545,11 @@ function submitCandidateAction(actionType: string, tile: string | null) {
               {{ tile }}
             </button>
           </div>
+          <div class="discard-row" v-if="game.selfSeat.melds.length > 0">
+            <span v-for="(meld, index) in game.selfSeat.melds" :key="`self-meld-${meld}-${index}`" class="discard-chip">
+              {{ meld }}
+            </span>
+          </div>
         </div>
       </article>
 
@@ -610,6 +617,12 @@ function submitCandidateAction(actionType: string, tile: string | null) {
               </span>
               <span v-if="seat.discardTiles.length === 0" class="empty-copy">暂无弃牌</span>
             </div>
+            <div class="discard-row">
+              <span v-for="(meld, index) in seat.melds" :key="`${seat.seatNo}-meld-${meld}-${index}`" class="discard-chip">
+                {{ meld }}
+              </span>
+              <span v-if="seat.melds.length === 0" class="empty-copy">暂无副露</span>
+            </div>
           </article>
         </div>