Validate image dimensions before sending to prevent API errors

[ethanndickson]

Problem

When users upload large images (>2000px) and have >20 images in a conversation, Anthropic rejects the request:

At least one of the image dimensions exceed max allowed size for many-image requests: 2000 pixels

This error persists in history, blocking subsequent messages until manually removed.

Proposed Solution

Add frontend validation to auto-resize images on paste/drop:

1. Extract dimensions when creating ImageAttachment (decode base64 → Image element → naturalWidth/Height)
2. Auto-resize to 2000px if larger (canvas downscale, preserve aspect ratio)
3. Show indicator when image was resized: "Image resized from 4000×3000 to 2000×1500"

Provider Limits Reference

Provider	Single Image	Many-Image (>20)
Anthropic	8000px	2000px
OpenAI	2000px	2000px
Google	8000px	8000px