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/attachments/types.ts

@@ -1,4 +1,8 @@
-import type { AgentAttachment, GitHubSearchItem } from "@getpaseo/protocol/messages";
+import type {
+  AgentAttachment,
+  GitHubSearchItem,
+  UploadedFileAttachment,
+} from "@getpaseo/protocol/messages";
 
 export type AttachmentStorageType = "web-indexeddb" | "desktop-file" | "native-file";
 
@@ -61,6 +65,7 @@ export type PullRequestContextAttachment =
 
 export type UserComposerAttachment =
   | { kind: "image"; metadata: AttachmentMetadata }
+  | { kind: "file"; attachment: UploadedFileAttachment }
   | { kind: "github_issue"; item: GitHubSearchItem }
   | { kind: "github_pr"; item: GitHubSearchItem };