fix: 투자 소수점 매수 금액 입력 모드 추가

- 매매 모달에 '입력 방식: 단가/금액' 선택 추가
- 금액 모드: 수량+총금액 입력 → 단가=round(금액/수량) 자동계산 (소수점 매수)
- 예) 0.6주를 10,000원에 매수 시 6,000원이 아니라 10,000원으로 정확히 기록
- 단가/금액 상호 미리보기 표시

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/views/account/InvestPortfolio.vue

@@ -1,5 +1,5 @@
 <script setup>
-import { onMounted, reactive, ref } from 'vue'
+import { computed, onMounted, reactive, ref } from 'vue'
 import { accountApi } from '@/api/accountApi'
 import IconBtn from '@/components/ui/IconBtn.vue'
 
@@ -19,10 +19,25 @@ const hError = ref(null)
 // 매매 모달
 const tradeModal = ref(false)
 const tradeHolding = ref(null)
-const tForm = reactive({ tradeType: 'BUY', tradeDate: '', quantity: null, price: null, fee: null })
+// inputMode: 'PRICE'(수량×단가) | 'AMOUNT'(수량+총금액 → 단가 자동계산, 소수점 매수용)
+const tForm = reactive({ tradeType: 'BUY', tradeDate: '', quantity: null, price: null, amount: null, fee: null, inputMode: 'PRICE' })
 const tSubmitting = ref(false)
 const tError = ref(null)
 
+const tradeQty = computed(() => Number(tForm.quantity) || 0)
+// 실제 저장에 쓰는 단가(원/주). 금액 모드면 총금액÷수량을 반올림.
+const tradePrice = computed(() => {
+  if (tForm.inputMode === 'AMOUNT') {
+    return tradeQty.value > 0 ? Math.round((Number(tForm.amount) || 0) / tradeQty.value) : 0
+  }
+  return Number(tForm.price) || 0
+})
+// 표시용 총 거래금액
+const tradeAmount = computed(() => {
+  if (tForm.inputMode === 'AMOUNT') return Number(tForm.amount) || 0
+  return Math.round(tradeQty.value * (Number(tForm.price) || 0))
+})
+
 // 매매이력 드롭다운
 const openTradesId = ref(null)
 const tradesByHolding = reactive({})
@@ -107,7 +122,7 @@ async function removeHolding(h) {
 /* ===== 매매 ===== */
 function openTrade(h, type) {
   tradeHolding.value = h
-  Object.assign(tForm, { tradeType: type, tradeDate: todayStr(), quantity: null, price: h.currentPrice ?? null, fee: null })
+  Object.assign(tForm, { tradeType: type, tradeDate: todayStr(), quantity: null, price: h.currentPrice ?? null, amount: null, fee: null, inputMode: 'PRICE' })
   tError.value = null
   tradeModal.value = true
 }
@@ -117,7 +132,12 @@ async function submitTrade() {
     tError.value = '수량을 입력하세요.'
     return
   }
-  if (tForm.price == null || tForm.price < 0) {
+  if (tForm.inputMode === 'AMOUNT') {
+    if (!tForm.amount || tForm.amount <= 0) {
+      tError.value = '금액을 입력하세요.'
+      return
+    }
+  } else if (tForm.price == null || tForm.price < 0) {
     tError.value = '단가를 입력하세요.'
     return
   }
@@ -126,7 +146,7 @@ async function submitTrade() {
     tradeType: tForm.tradeType,
     tradeDate: tForm.tradeDate,
     quantity: Number(tForm.quantity),
-    price: Number(tForm.price),
+    price: tradePrice.value,
     fee: tForm.fee ? Number(tForm.fee) : 0,
   }
   try {
@@ -266,7 +286,20 @@ onMounted(load)
               </label>
               <label>거래일<input v-model="tForm.tradeDate" type="date" :disabled="tSubmitting" /></label>
               <label>수량(주)<input v-model.number="tForm.quantity" type="number" min="0" step="any" inputmode="decimal" placeholder="예: 0.5 / 1.25 (소수점 가능)" :disabled="tSubmitting" /></label>
-              <label>단가(원)<input v-model.number="tForm.price" type="number" min="0" :disabled="tSubmitting" /></label>
+              <label>입력 방식
+                <select v-model="tForm.inputMode" :disabled="tSubmitting">
+                  <option value="PRICE">단가로 입력</option>
+                  <option value="AMOUNT">금액으로 입력 (소수점 매수)</option>
+                </select>
+              </label>
+              <label v-if="tForm.inputMode === 'PRICE'">단가(원)
+                <input v-model.number="tForm.price" type="number" min="0" :disabled="tSubmitting" />
+                <small v-if="tradeQty > 0 && tForm.price" class="pf-hint">총 약 {{ won(tradeAmount) }}원</small>
+              </label>
+              <label v-else>금액(총, 원)
+                <input v-model.number="tForm.amount" type="number" min="0" placeholder="이 금액어치 매매" :disabled="tSubmitting" />
+                <small v-if="tradeQty > 0 && tForm.amount" class="pf-hint">단가 약 {{ won(tradePrice) }}원</small>
+              </label>
               <label>수수료/세금<input v-model.number="tForm.fee" type="number" min="0" placeholder="(선택)" :disabled="tSubmitting" /></label>
               <p v-if="tForm.tradeType === 'SELL'" class="pf-hint">보유 {{ shares(tradeHolding?.quantity) }}주 · 평단 {{ won(tradeHolding?.avgPrice) }}</p>
               <p v-if="tError" class="pf-msg err">{{ tError }}</p>