perf(web): skip base64 for oversized image candidates

[tarik02]

What Changed

Backports pingdotgg#5220.

• Check the estimated base64 data URL length from the encoded blob size.
• Skip base64 conversion for candidates that already exceed the size budget.
• Restore bounded thread subscription replay lost during the fork’s last upstream sync so the existing server tests pass.

Why

Image compression converted every canvas candidate to base64 before checking whether it fit. Large images repeated this work across each quality and downscale attempt.

The fork sync left thread subscriptions replaying to Number.MAX_SAFE_INTEGER instead of the captured head, which broke three existing tests.

Checklist

• This PR is small and focused
• I explained what changed and why
• UI screenshots are not applicable
• Animation video is not applicable