feat: preview image attachments in the chat composer

[wyx20021207-create]

Summary

• Reuse the existing ImagePreview modal for attachments in the chat composer, before sending.
• Preserve thumbnail sizing and keep the remove button separate from the preview trigger.
• Reuse keyboard activation, Escape/backdrop dismissal, and focus restoration from the shared component.

Testing

• Shared image-preview regression tests pass.
• npm run lint and TypeScript tsc --noEmit pass in the development checkout.
• Ran npm test on Windows and compared it with an untouched upstream/main checkout at 0d1df12: both have the same 14 failing tests (including Windows symlink/shell limitations and existing component assertions). No additional failing tests were observed.
• Production build and browser E2E were not run locally.

Manual verification

1. Paste, drag, or attach an image in the composer.
2. Click the thumbnail to open the modal; close with Escape, backdrop, or the close button.
3. Use the remove button and confirm it removes the attachment without opening the modal.

This PR is independent of the local-path preview feature.