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/en.ts

@@ -97,12 +97,15 @@ export const en = {
     },
     attachments: {
       addImage: "Add image",
+      addFile: "Upload file",
       addIssueOrPr: "Add issue or PR",
       dropImagesHere: "Drop images here",
+      dropFilesHere: "Drop files here",
       editQueuedMessage: "Edit queued message",
       sendQueuedMessageNow: "Send queued message now",
       openImage: "Open image attachment",
       removeImage: "Remove image attachment",
+      removeFile: "Remove file attachment",
       openGithub: "Open {{kind}} #{{number}}",
       removeGithub: "Remove {{kind}} #{{number}}",
       browserElement: "Element · {{tag}}",
@@ -117,6 +120,8 @@ export const en = {
       noHostSelected: "No host selected",
       initialPromptRequired: "Initial prompt is required",
       alreadyLoading: "Already loading",
+      uploadFailed: "Failed to upload file",
+      fileTooLarge: "{{fileName}} is too large (max {{size}})",
     },
     clientCommands: {
       archiveAgent: "Archive the current agent",