feat(app): add drag-and-drop file upload and file picker to composer (#1474)

- Desktop web: drag-and-drop non-image files onto the chat pane to upload
- Desktop web: file picker option in the composer attachment sheet
- Uploaded files are sent to the LLM as filename references in a tmp directory
- New useFilePicker hook for cross-platform file picking
- Extended useFileDropZone to support generic file drops
- Added UploadedFileAttachment type to protocol and composer attachment system
- Fixed adaptive modal sheet closing on backdrop press in selection mode

packages/app/src/i18n/resources/zh-CN.ts

@@ -99,12 +99,15 @@ export const zhCN: TranslationResources = {
     },
     attachments: {
       addImage: "添加图片",
+      addFile: "Upload file",
       addIssueOrPr: "添加 issue 或 PR",
       dropImagesHere: "将图片拖放到这里",
+      dropFilesHere: "Drop files here",
       editQueuedMessage: "编辑排队消息",
       sendQueuedMessageNow: "立即发送排队消息",
       openImage: "打开图片附件",
       removeImage: "移除图片附件",
+      removeFile: "Remove file attachment",
       openGithub: "打开 {{kind}} #{{number}}",
       removeGithub: "移除 {{kind}} #{{number}}",
       browserElement: "元素 · {{tag}}",
@@ -119,6 +122,8 @@ export const zhCN: TranslationResources = {
       noHostSelected: "未选择 Host",
       initialPromptRequired: "初始 prompt 必填",
       alreadyLoading: "正在加载",
+      uploadFailed: "Failed to upload file",
+      fileTooLarge: "{{fileName}} is too large (max {{size}})",
     },
     clientCommands: {
       archiveAgent: "归档当前 Agent",