feat: 新增账单导出 Excel 功能

- 后端新增 /api/bills/export 接口，支持当前筛选条件导出全部记录
- 使用 excelize 库生成 xlsx 格式文件
- 前端账单管理页面添加导出按钮
- 更新 Go 版本到 1.24 以支持 excelize 依赖

web/src/lib/api.ts

@@ -316,6 +316,46 @@ export async function fetchBills(params: FetchBillsParams = {}): Promise<BillsRe
   return response.json();
 }
 
+// 导出账单为 Excel
+export async function exportBills(params: FetchBillsParams = {}): Promise<void> {
+  const searchParams = new URLSearchParams();
+  
+  if (params.start_date) searchParams.set('start_date', params.start_date);
+  if (params.end_date) searchParams.set('end_date', params.end_date);
+  if (params.category) searchParams.set('category', params.category);
+  if (params.type) searchParams.set('type', params.type);
+  if (params.income_expense) searchParams.set('income_expense', params.income_expense);
+  
+  const queryString = searchParams.toString();
+  const url = `${API_BASE}/api/bills/export${queryString ? '?' + queryString : ''}`;
+  
+  const response = await apiFetch(url);
+  
+  if (!response.ok) {
+    throw new Error(`HTTP ${response.status}`);
+  }
+  
+  const blob = await response.blob();
+  const contentDisposition = response.headers.get('Content-Disposition');
+  let filename = `bills_${new Date().toISOString().slice(0, 10)}.xlsx`;
+  
+  if (contentDisposition) {
+    const match = contentDisposition.match(/filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/);
+    if (match && match[1]) {
+      filename = match[1].replace(/['"]/g, '');
+    }
+  }
+  
+  const objectUrl = window.URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = objectUrl;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  window.URL.revokeObjectURL(objectUrl);
+}
+
 // 手动输入账单数据
 export interface ManualBillInput {
   time: string;

web/src/routes/bills/+page.svelte

@@ -15,8 +15,9 @@
   import BillDetailDrawer from '$lib/components/analysis/BillDetailDrawer.svelte';
   import { cleanedBillToUIBill, type UIBill } from '$lib/models/bill';
   import { categories } from '$lib/data/categories';
-  import { formatLocalDate, formatDateTime } from '$lib/utils';
-  import Loader2 from '@lucide/svelte/icons/loader-2';
+import { formatLocalDate, formatDateTime } from '$lib/utils';
+import { exportBills } from '$lib/api';
+import Loader2 from '@lucide/svelte/icons/loader-2';
   import AlertCircle from '@lucide/svelte/icons/alert-circle';
   import Search from '@lucide/svelte/icons/search';
   import Receipt from '@lucide/svelte/icons/receipt';
@@ -26,9 +27,10 @@
   import Filter from '@lucide/svelte/icons/filter';
   import ChevronLeft from '@lucide/svelte/icons/chevron-left';
   import ChevronRight from '@lucide/svelte/icons/chevron-right';
-  import RefreshCw from '@lucide/svelte/icons/refresh-cw';
-  import Plus from '@lucide/svelte/icons/plus';
-  import List from '@lucide/svelte/icons/list';
+import RefreshCw from '@lucide/svelte/icons/refresh-cw';
+import Plus from '@lucide/svelte/icons/plus';
+import List from '@lucide/svelte/icons/list';
+import Download from '@lucide/svelte/icons/download';
   
   // 状态
   let isLoading = $state(false);
@@ -221,6 +223,29 @@
       totalIncome = Math.max(0, totalIncome - deleted.amount);
     }
   }
+
+  // 导出 Excel
+  let isExporting = $state(false);
+  let exportError = $state('');
+  
+  async function handleExport() {
+    isExporting = true;
+    exportError = '';
+    
+    try {
+      await exportBills({
+        start_date: startDate || undefined,
+        end_date: endDate || undefined,
+        category: filterCategory || undefined,
+        type: filterBillType || undefined,
+        income_expense: filterIncomeExpense || undefined,
+      });
+    } catch (err) {
+      exportError = err instanceof Error ? err.message : '导出失败';
+    } finally {
+      isExporting = false;
+    }
+  }
 </script>
 
 <svelte:head>
@@ -240,6 +265,10 @@
           <RefreshCw class="mr-2 h-4 w-4 {isLoading ? 'animate-spin' : ''}" />
           刷新
         </Button>
+        <Button variant="outline" onclick={handleExport} disabled={isExporting || totalRecords === 0}>
+          <Download class="mr-2 h-4 w-4 {isExporting ? 'animate-spin' : ''}" />
+          导出 Excel
+        </Button>
       {/if}
     </div>
   </div>