bug(desktop): pasting a Finder folder stages it as a file and makes send fail

### What happened

On macOS, copying a directory in Finder and pasting it into the Desktop composer creates a normal file-attachment chip. The chip looks sendable, but submitting the message fails with the generic send-failure toast ("Failed to send" / "Please try again").

Expected behavior: a pasted directory should either become the existing Host-bound folder reference used by **Reference folder**, or be rejected during paste with a clear localized explanation. It must not be staged as a normal file attachment that can only fail during submission.

This should not recursively upload or eagerly enumerate the directory.

### How to reproduce

1. Open Maka Desktop and an agent session with no turn running.
2. In macOS Finder, select a folder and press Cmd+C.
3. Focus the Desktop composer and press Cmd+V.
4. Observe that the folder appears as a normal file attachment.
5. Enter any text if needed and click Send.
6. Observe that submission fails with the generic send-failure toast.

### Environment

- Maka version or commit: observed in a downstream Desktop build at `3fe9ec9`; source inspection confirms the same path is still present in upstream `main` at `f109ccde9`
- OS and version: Darwin 25.6.0 (macOS, arm64)
- Surface: Desktop
- Electron: 43.4.1
- Chrome: 150.0.7871.224
- Node.js: 24.18.1
- Locale: zh-CN

### Logs, screenshots, or additional context

The visible toast is:

- Title: `发送失败`
- Description: `消息暂时无法发送，请稍后重试。`

The diagnostic report does not include the underlying renderer/preload exception. The source path explains why the folder first appears valid and only fails at send time:

- `packages/ui/src/composer.tsx`: `onInputFiles` forwards every clipboard `File` to `onAttachFilePaths` without distinguishing a directory.
- `packages/ui/src/use-composer-attachments.ts`: `sniffFileMimeType` catches a failed `file.slice(...).arrayBuffer()` read and still stages the item as an attachment.
- `apps/desktop/src/renderer/attachment-preflight.ts`: preflight checks only count and byte size, so a directory-shaped `File` is not rejected.
- `apps/desktop/src/preload/attachment-ingest-payload.ts`: submission later calls `file.arrayBuffer()` without mapping an unreadable/directory source to an expected attachment error, so the exception falls through to the generic send failure.

Related but not duplicate:

- #3994 defined directory context behavior.
- #4097 added the explicit **Reference folder** picker and Host-bound directory references, but the Finder paste path still goes through ordinary attachment ingestion.
- #4188 covered pasted images while a turn is running, not pasted directories.

A focused regression test could paste a directory-shaped clipboard item and assert that it is never staged as an ordinary attachment or allowed to reach `encodeIngestItems` as a file.

AI assistance: OpenAI Codex inspected the source paths, searched existing Issues and PRs, and drafted this report.