Improve feed and image loading performance

[hjiangsu]

Pull Request Description

This PR applies a bunch of small changes to improve feed/image loading performance. This is mostly noticeable with instances using AVIF images as thumbnails. Some of the changes include:

• Adding equatable to internal Thunder models (for equality checking)
• Improvements to AVIF image dimension logic using flutter_avif package
• Improvements to image rendering for AVIF images

Issue Being Fixed

Issue Number: #2009

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?