支持客户、对话记录与统计报告 CSV 导出

- 新增 /customers/export、/sessions/export、/statistics/export
- 沿用列表筛选与角色可见范围，UTF-8 BOM 便于 Excel 打开
- 前端三处导出按钮接入真实下载

server/internal/handler/export_csv.go

@@ -0,0 +1,70 @@
+package handler
+
+import (
+	"encoding/csv"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+const exportMaxRows = 5000
+
+func csvEscape(s string) string {
+	return strings.ReplaceAll(s, "\r\n", " ")
+}
+
+func writeCSVResponse(c *gin.Context, filename string, header []string, rows [][]string) {
+	if !strings.HasSuffix(strings.ToLower(filename), ".csv") {
+		filename += ".csv"
+	}
+	// 兼容 Excel 中文：UTF-8 BOM
+	c.Header("Content-Type", "text/csv; charset=utf-8")
+	c.Header("Content-Disposition", fmt.Sprintf("attachment; filename*=UTF-8''%s", url.PathEscape(filename)))
+	c.Status(http.StatusOK)
+
+	if _, err := c.Writer.Write([]byte{0xEF, 0xBB, 0xBF}); err != nil {
+		return
+	}
+	w := csv.NewWriter(c.Writer)
+	_ = w.Write(header)
+	for _, row := range rows {
+		for i := range row {
+			row[i] = csvEscape(row[i])
+		}
+		_ = w.Write(row)
+	}
+	w.Flush()
+}
+
+func formatCSVTime(t time.Time) string {
+	if t.IsZero() {
+		return ""
+	}
+	return t.In(time.Local).Format("2006-01-02 15:04:05")
+}
+
+func formatCSVTimePtr(t *time.Time) string {
+	if t == nil {
+		return ""
+	}
+	return formatCSVTime(*t)
+}
+
+func formatUintPtr(v *uint) string {
+	if v == nil {
+		return ""
+	}
+	return strconv.FormatUint(uint64(*v), 10)
+}
+
+func formatIntPtr(v *int) string {
+	if v == nil {
+		return ""
+	}
+	return strconv.Itoa(*v)
+}