拆分 main 功能模块

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

message/image.go

@@ -0,0 +1,50 @@
+package message
+
+import (
+	"encoding/base64"
+	"errors"
+	"strings"
+
+	"aichat/utils"
+)
+
+const maxImageSize = 4 * 1024 * 1024
+
+var allowedImageTypes = map[string]bool{
+	"image/jpeg": true,
+	"image/png":  true,
+	"image/webp": true,
+	"image/gif":  true,
+}
+
+func normalizeImageDataURI(raw string) (string, error) {
+	comma := strings.Index(raw, ",")
+	if comma < 0 {
+		return "", errors.New("图片 base64 数据格式错误")
+	}
+
+	meta := strings.ToLower(strings.TrimSpace(raw[5:comma]))
+	payload := strings.TrimSpace(raw[comma+1:])
+	if payload == "" {
+		return "", errors.New("图片 base64 数据不能为空")
+	}
+	parts := strings.Split(meta, ";")
+	if len(parts) < 2 || !utils.Contains(parts[1:], "base64") {
+		return "", errors.New("图片 data URI 必须使用 base64 编码")
+	}
+
+	mime := parts[0]
+	if !allowedImageTypes[mime] {
+		return "", errors.New("图片格式不支持，仅支持 jpeg/png/webp/gif")
+	}
+
+	decoded, err := base64.StdEncoding.DecodeString(payload)
+	if err != nil {
+		return "", errors.New("图片 base64 数据无效")
+	}
+	if len(decoded) > maxImageSize {
+		return "", errors.New("图片过大，请选择小于 4MB 的图片")
+	}
+
+	return "data:" + mime + ";base64," + payload, nil
+}