Add image attachments to the chat demo

[SamSamskies]

Summary

• Add attach / paste / drop image support to the chat demo when getFeatures().imageInput is available (jpeg, png, webp, gif).
• Send multimodal user content with image parts, show attachment previews in the composer and in sent messages.
• Harden paste and drag/drop so unsupported clipboard images keep text paste, streaming drops do not navigate away, and failed sends keep pending attachments with the draft.

Test plan

• With Bridge + vision-capable injector: attach images via button, paste, and drag/drop; send and confirm they appear in the user bubble and reach the model
• Paste plain text alone still works; paste with unsupported image type (e.g. SVG) still pastes text
• Drop a file while a reply is streaming — page should not navigate away
• Fail availability / image input unavailable with pending attachments — draft text and thumbnails should remain
• Without image input: Attach control stays disabled; text-only chat still works

Made with Cursor