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;