feat: P6 후속 4 — 출금 폼 잔여 한도 미리 표시

- 백엔드 GET /api/mobile/me/settle-masters/{id}/withdrawable-balance
  · 응답 {settleId, netAmount, usedAmount, remainingAmount}
  · 본인 settle_master 검증 후 net - sumActiveAmountByMaster 반환
  · createWithdrawRequest 가 동일 조회 로직 재사용 (중복 제거)
- PWA WithdrawCreate: settleMaster 선택 시 잔액 카드 표시 + amount 클라이언트 사전 검증

검증: ga-api compileJava BUILD SUCCESSFUL / ga-mobile-pwa tsc --noEmit 0 오류
(라이브 e2e 는 DB/docker 미가동으로 차단 — 환경 복구 후 스모크 예정)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ga-api/src/main/java/com/ga/api/controller/mobile/MobileMeController.java

@@ -3,6 +3,7 @@ package com.ga.api.controller.mobile;
 import com.ga.api.service.mobile.MobileMeService;
 import com.ga.api.service.mobile.MobileMeService.MeSummary;
 import com.ga.api.service.mobile.MobileMeService.MobileWithdrawSaveReq;
+import com.ga.api.service.mobile.MobileMeService.WithdrawableBalance;
 import com.ga.common.model.ApiResponse;
 import com.ga.common.model.PageResponse;
 import com.ga.core.vo.notice.UserNotificationResp;
@@ -69,6 +70,11 @@ public class MobileMeController {
         return ApiResponse.ok(service.withdrawableSettleMasters());
     }
 
+    @GetMapping("/settle-masters/{settleMasterId}/withdrawable-balance")
+    public ApiResponse<WithdrawableBalance> withdrawableBalance(@PathVariable Long settleMasterId) {
+        return ApiResponse.ok(service.withdrawableBalance(settleMasterId));
+    }
+
     @GetMapping("/withdraw-requests")
     public ApiResponse<PageResponse<WithdrawRequestResp>> withdrawRequests(
             @ModelAttribute WithdrawRequestSearchParam param) {

ga-api/src/main/java/com/ga/api/service/mobile/MobileMeService.java

@@ -168,6 +168,28 @@ public class MobileMeService {
         withdrawRequestService.cancel(requestId);
     }
 
+    /**
+     * 본인 정산 마스터의 출금 가능 잔액 조회 — 출금 신청 폼 미리 표시용.
+     * 본인 소유가 아니면 FORBIDDEN, 없으면 NOT_FOUND.
+     */
+    @Transactional(readOnly = true)
+    public WithdrawableBalance withdrawableBalance(Long settleMasterId) {
+        Long agentId = requireAgentId();
+        var master = settleMasterMapper.selectById(settleMasterId);
+        if (master == null) throw new BizException(ErrorCode.NOT_FOUND, "정산 마스터를 찾을 수 없습니다");
+        if (!agentId.equals(master.getAgentId())) throw new BizException(ErrorCode.FORBIDDEN);
+
+        BigDecimal netAmount = master.getNetAmount() != null ? master.getNetAmount() : BigDecimal.ZERO;
+        BigDecimal used = withdrawMapper.sumActiveAmountByMaster(settleMasterId);
+        if (used == null) used = BigDecimal.ZERO;
+        return WithdrawableBalance.builder()
+                .settleId(settleMasterId)
+                .netAmount(netAmount)
+                .usedAmount(used)
+                .remainingAmount(netAmount.subtract(used))
+                .build();
+    }
+
     /**
      * 본인 출금 신청 등록. agentId / requestedBy 는 토큰에서 강제 결정.
      * 클라이언트가 보낸 값은 무시한다.
@@ -175,22 +197,14 @@ public class MobileMeService {
     @Transactional
     public Long createWithdrawRequest(MobileWithdrawSaveReq req) {
         Long userId = currentUserId();
-        Long agentId = requireAgentId();
 
-        // 1) settle_master 본인 소유 + net_amount 확보
-        var master = settleMasterMapper.selectById(req.getSettleMasterId());
-        if (master == null) throw new BizException(ErrorCode.NOT_FOUND, "정산 마스터를 찾을 수 없습니다");
-        if (!agentId.equals(master.getAgentId())) throw new BizException(ErrorCode.FORBIDDEN);
-
-        // 2) 잔여 한도 = net_amount - 활성 출금합계
-        BigDecimal netAmount = master.getNetAmount() != null ? master.getNetAmount() : BigDecimal.ZERO;
-        BigDecimal used = withdrawMapper.sumActiveAmountByMaster(req.getSettleMasterId());
-        if (used == null) used = BigDecimal.ZERO;
-        BigDecimal remaining = netAmount.subtract(used);
-        if (req.getAmount().compareTo(remaining) > 0) {
+        // 본인 소유 검증 + 잔여 한도 = net_amount - 활성 출금합계 (조회 로직 재사용)
+        WithdrawableBalance balance = withdrawableBalance(req.getSettleMasterId());
+        if (req.getAmount().compareTo(balance.getRemainingAmount()) > 0) {
             throw new BizException(ErrorCode.INVALID_PARAMETER,
-                    "출금 가능 잔액(" + remaining + ")을 초과합니다");
+                    "출금 가능 잔액(" + balance.getRemainingAmount() + ")을 초과합니다");
         }
+        Long agentId = requireAgentId();
 
         WithdrawRequestSaveReq inner = new WithdrawRequestSaveReq();
         inner.setSettleMasterId(req.getSettleMasterId());
@@ -234,6 +248,16 @@ public class MobileMeService {
         private int unreadNotificationCount;
     }
 
+    /** 출금 가능 잔액 — 폼 미리 표시용. */
+    @Data
+    @Builder
+    public static class WithdrawableBalance {
+        private Long settleId;
+        private BigDecimal netAmount;
+        private BigDecimal usedAmount;
+        private BigDecimal remainingAmount;
+    }
+
     /** 모바일 출금 신청 — 토큰에서 agentId/requestedBy 강제. */
     @Data
     public static class MobileWithdrawSaveReq {