feat: add media attachment support for admin ticket replies

Admin can now attach photos, videos, and documents when replying to
tickets. Upload happens on file selection via existing media upload
endpoint. Includes attachment preview with type-specific icons,
upload progress, error handling, race condition protection, and
proper blob URL lifecycle management.

Added video player rendering for video attachments in message thread.
Updated ticket list preview to show [media_type] for media-only messages.

src/locales/zh.json

@@ -1617,6 +1617,12 @@
       "adminLabel": "管理员",
       "replyPlaceholder": "输入您的回复...",
       "sendReply": "发送回复",
+      "attachMedia": "附加媒体",
+      "invalidFileType": "文件类型无效。请使用图片、视频 (MP4, WebM) 或文档 (PDF, DOC, TXT, ZIP)。",
+      "fileTooLarge": "文件太大。最大 10MB。",
+      "uploadFailed": "上传文件失败",
+      "uploading": "上传中...",
+      "uploadComplete": "已上传",
       "settings": "设置",
       "settingsSubtitle": "配置工单系统和SLA",
       "settingsLoadError": "加载设置失败",