fix: reject unsupported image formats before API call, fix scroll layout jumps (#351, #844)

[MrRealORG]

Summary

Fixes two bugs:

#351: Unsupported image format causes repeated 400 errors

When an unsupported image format (e.g. .ico, .tiff) is attached to a conversation, the MiMo API rejects it with a 400 error. Because the invalid media stays in the message history, every subsequent turn re-sends it, causing the same error repeatedly and making the conversation unusable.

Fix: Added a SUPPORTED_IMAGE_MIMES allowlist in util/media.ts (bmp, gif, jpeg, png, webp) and a pre-send filter in provider/transform.ts that converts unsupported image formats to a clear text error before they reach the API. This prevents the 400 error entirely.

#844: Scroll causes right-side display glitch

CSS content-visibility: auto without matching contain-intrinsic-size causes the browser to estimate element sizes as 0px when off-screen, producing layout jumps during scrolling.

Fix: Added contain-intrinsic-size to all CSS rules using content-visibility: auto:

• message-part.css: auto 500px
• basic-tool.css: auto 200px
• file.css: auto 300px

Test Plan

• Attach an .ico file — verify clear error instead of 400 loop
• Attach a .png file — verify it still works
• Scroll in long conversation — verify no display glitches

Closes #351
Closes #844