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

backend/src/test/java/com/xuezhanmaster/game/event/GameEventTest.java

@@ -94,4 +94,21 @@ class GameEventTest {
                 .doesNotContainKey("sourceSeatNo")
                 .doesNotContainKey("tile");
     }
+
+    @Test
+    void shouldAllowGangEventWithoutSourceSeatForConcealedGang() {
+        GameEvent concealedGang = GameEvent.responseActionDeclared(
+                "game-1",
+                GameEventType.GANG_DECLARED,
+                0,
+                null,
+                "3万"
+        );
+
+        assertThat(concealedGang.eventType()).isEqualTo(GameEventType.GANG_DECLARED);
+        assertThat(concealedGang.payload())
+                .containsEntry("actionType", "GANG")
+                .containsEntry("tile", "3万")
+                .doesNotContainKey("sourceSeatNo");
+    }
 }