feat: 예산 월별 분리 + 전월/다음달 복사 버튼

- budgets/createBudget에 year/month 전달, copyBudget 추가
- BudgetView: '전월 예산 가져오기' / '다음 달로 복사' 버튼
- 백업 export는 현재 월 예산을 담음

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

src/views/account/BudgetView.vue

@@ -127,7 +127,7 @@ async function load() {
   error.value = null
   try {
     const params = { year: year.value, month: month.value }
-    const [st, raw] = await Promise.all([accountApi.budgetStatus(params), accountApi.budgets()])
+    const [st, raw] = await Promise.all([accountApi.budgetStatus(params), accountApi.budgets(params)])
     statuses.value = st
     budgetsRaw.value = raw
   } catch (e) {
@@ -154,6 +154,46 @@ function nextMonth() {
   loadIncome()
 }
 
+// ===== 월별 예산 복사 =====
+const copying = ref(false)
+const ym = (y, m) => `${y}.${String(m).padStart(2, '0')}`
+async function copyToNextMonth() {
+  if (copying.value || !budgetsRaw.value.length) return
+  const to = month.value === 12 ? { y: year.value + 1, m: 1 } : { y: year.value, m: month.value + 1 }
+  const ok = await dialog.confirm(
+    `${ym(year.value, month.value)} 예산을 ${ym(to.y, to.m)}(다음 달)로 복사할까요?\n다음 달의 같은 분류 예산은 덮어씁니다.`,
+    { title: '다음 달로 복사' },
+  )
+  if (!ok) return
+  copying.value = true
+  try {
+    await accountApi.copyBudget({ fromYear: year.value, fromMonth: month.value, toYear: to.y, toMonth: to.m })
+    await dialog.alert(`${ym(to.y, to.m)}로 예산을 복사했어요.`)
+  } catch (e) {
+    error.value = e.response?.data?.message || '복사에 실패했습니다.'
+  } finally {
+    copying.value = false
+  }
+}
+async function copyFromPrevMonth() {
+  if (copying.value) return
+  const prev = month.value === 1 ? { y: year.value - 1, m: 12 } : { y: year.value, m: month.value - 1 }
+  const ok = await dialog.confirm(
+    `전월(${ym(prev.y, prev.m)}) 예산을 ${ym(year.value, month.value)}로 가져올까요?\n이 달의 같은 분류 예산은 덮어씁니다.`,
+    { title: '전월 예산 복사' },
+  )
+  if (!ok) return
+  copying.value = true
+  try {
+    await accountApi.copyBudget({ fromYear: prev.y, fromMonth: prev.m, toYear: year.value, toMonth: month.value })
+    await load()
+  } catch (e) {
+    error.value = e.response?.data?.message || '복사에 실패했습니다.'
+  } finally {
+    copying.value = false
+  }
+}
+
 function openCreate() {
   editId.value = null
   Object.assign(form, {
@@ -213,7 +253,7 @@ async function submit() {
   submitting.value = true
   try {
     if (editId.value) await accountApi.updateBudget(editId.value, payload)
-    else await accountApi.createBudget(payload)
+    else await accountApi.createBudget(payload, { year: year.value, month: month.value })
     formOpen.value = false
     await load()
   } catch (e) {
@@ -252,6 +292,12 @@ onMounted(() => {
       </div>
     </div>
 
+    <!-- 월별 예산 복사 -->
+    <div class="copy-actions">
+      <button type="button" class="copy-btn" :disabled="copying" @click="copyFromPrevMonth">↤ 전월 예산 가져오기</button>
+      <button type="button" class="copy-btn" :disabled="copying || !budgetsRaw.length" @click="copyToNextMonth">다음 달로 복사 ↦</button>
+    </div>
+
     <!-- 월 예상 수입 vs 총 예산 -->
     <div class="income-panel">
       <div class="inc-input">
@@ -431,6 +477,26 @@ button.primary {
   text-align: center;
 }
 /* 예상 수입 vs 예산 */
+.copy-actions {
+  display: flex;
+  gap: 0.5rem;
+  justify-content: flex-end;
+  flex-wrap: wrap;
+  margin: -0.4rem 0 1rem;
+}
+.copy-btn {
+  padding: 0.4rem 0.8rem;
+  border: 1px solid var(--color-border);
+  border-radius: 6px;
+  background: var(--color-background-soft);
+  color: var(--color-text);
+  font-size: 0.82rem;
+  cursor: pointer;
+}
+.copy-btn:disabled {
+  opacity: 0.45;
+  cursor: default;
+}
 .income-panel {
   border: 1px solid var(--color-border);
   border-radius: 8px;