fix(web): render image attachments in chat messages

[ericleepi314]

Description

Image attachments currently become plain [Image #N] text after sending, and reopening a saved conversation drops its image blocks. Render responsive, rounded image previews above the user message so attachments remain visible after sending and reloading.

• Preserve preview bytes in live and queued user messages, and restore images from saved content blocks, including image-only turns.
• Hide leading attachment markers when a preview is available and omit backend image metadata from captions; preserve inline references and the original submitted text.
• Keep multiple composer previews valid and release their object URLs after sending or unmounting.

Validation

• npm run typecheck — passed.
• npm run test — full web suite passed.
• npm run build — passed.
• Headless Chrome with a mocked gateway: upload/send, reload, multiple attachments, image-only messages, and 390px viewport layout passed without page errors.

New Feature Submissions

• Explained the change and why it should be included.

All Submissions

Notify authors: @agentforce314, @ericleepi314