iOS: Respect Auto-Play Animated Images setting

[ganidande905]

Overview - Fixes #1924

This PR updates ImageAnimationMode.shouldAnimate to respect the iOS 17+ accessibility setting Auto-Play Animated Images, using Flutter’s AccessibilityFeatures.autoPlayAnimatedImages.

What changed

• Replace the iOS-specific gating that used reduceMotion with a check on autoPlayAnimatedImages (the setting intended for animated images/GIFs).

Testing

• Verified on iOS that uploaded GIFs/attachments (rendered via ZulipImage) stop animating when Auto-Play Animated Images is disabled, and animate when enabled.

Edge case / follow-up

• GIFs displayed via pasted URLs in message content still auto-play. That path doesn’t appear to consult ImageAnimationMode, so it bypasses this check. Handling inline URL GIFs would likely require threading ImageAnimationMode (or equivalent gating) into the inline image/link-preview renderer, which seems out of scope for this issue.

Screen Recording

The screen recording demonstrates the behaviour of animated images with the iOS Auto-Play Animated Images accessibility setting:

• Inline GIFs from pasted URLs (top two examples) and uploaded GIFs both animate normally when the setting is enabled.
• When Auto-Play Animated Images is disabled, animated images rendered via ZulipImage (including uploaded GIFs) stop animating.
  Inline GIFs from pasted URLs are included in the recording to show current behaviour; however, that rendering path does not consult ImageAnimationMode, so it does not yet respect the setting (as noted in the edge-case section above).

Screen.Recording.Feb.26.2026.1.1.mov

[ganidande905]

@chrisbobbe Could you please review this when convenient? The PR follows the contribution guidelines and all CI checks are passing. Thanks

[chrisbobbe]

Please remove the unrelated code formatting changes.

[ganidande905]

@chrisbobbe I’ve removed the unrelated formatting changes and kept the diff limited to the intended logic change. Please let me know if this looks good now.

[chrisbobbe]

Please update your commit message to match our style (it's missing a properly-formatted issue reference) and add a test for this new logic; see the project README.

It looks like the feature is new in iOS 17. What behavior do we offer users on older iOS versions? The earliest we support is 15.0 (from ios/Podfile). Probably falling back to that reduce-motion setting would be a reasonable behavior. We should also include test coverage for that case.

[ganidande905]

I’ll update the commit message and add tests for the new logic.
I’ll also update the implementation so that:

• iOS 17+ respects autoPlayAnimatedImages
• iOS 15–16 fall back to reduceMotion
  I’ll push an updated commit shortly.

[ganidande905]

Hi! I've pushed the latest changes addressing the feedback and updated the tests as well. Could you please take a look at the updated commits when you have time? Thanks!

[gnprice]

Thanks. Before we can fully review this PR, you'll need to revise it to have clear and coherent commits. See the resources linked from our README.

Pay attention also to making the code look and work similarly to existing code in the codebase. For example, note what our README says about check vs. expect.