Clicking an attachment chip in the composer now opens the same FilePreviewModal
the message viewer uses, instead of offering only download/remove. The chip
becomes clickable once the attachment has content (a local File, or an uploaded
blob for forwarded attachments) and the type is previewable.
- getFileContent prefers the in-memory File (no network round-trip) and falls
back to composerClient.fetchBlob for forwarded attachments (blobId only).
- Previewability (isFilePreviewable) and the open-in-new-tab safety gate are
handled inside the modal, so this adds no new egress/attack surface; the local
download path uses an <a download> (forces a save, never executes).
- No new dependencies and no new locale keys.
