feat(webui): support document attachments with ingress safeguards (#4771)

* feat: support document attachments in webui

* fix(webui): normalize document attachment MIME

* refactor(webui): move attachment policy out of channel

* fix(webui): reject oversized attachments before send

* fix(webui): align Portuguese attachment errors

* refactor(webui): separate ingress and transport limits

* fix(webui): reject malformed attachment payloads

webui/src/i18n/locales/zh-CN/common.json

@@ -908,7 +908,7 @@
         "edit": "编辑引导",
         "drag": "拖动排序"
       },
-      "attachImage": "添加图片",
+      "attachImage": "添加文件",
       "imageMode": {
         "label": "图片生成",
         "toggle": "切换图片生成模式",
@@ -1033,12 +1033,17 @@
       "encoding": "处理中…",
       "remove": "移除附件",
       "normalizedSizeHint": "{{orig}} → {{current}}（已自动压缩）",
+      "textTooLarge": "消息文本过大（最大 {{max}}）",
       "imageRejected": {
         "unsupported_type": "不支持的文件类型",
+        "empty_file": "不能附加空文件",
+        "too_many_attachments": "每条消息最多 {{max}} 个附件",
         "too_many_images": "每条消息最多 {{max}} 张图片",
         "magic_mismatch": "文件看起来不像真实的图片",
         "decode_failed": "无法解码这张图片",
-        "too_large": "图片太大，请换一张小一点的",
+        "too_large": "文件太大，请换一个小一点的",
+        "total_too_large": "附件总大小过大，请移除部分文件或使用更小的文件",
+        "transport_too_large": "该附件会超过网关的传输上限",
         "io": "无法读取该文件"
       },
       "goalStateCloseAria": "关闭目标",