Support gallery messages

[bxdxnn]

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[bxdxnn]

This a PoC implementation. Vibecoded as I'm not a Web dev.
It already works, but a better design for tile management needs to be figured out and I guess this is out of my scope.

[t3chguy]

It helps to provide screenshots. Your PR doesn't describe what it changes. The title is ambiguous. Sounds like it may do the same as #31932 which has had feedback from Design which probably also apply here #31932 (review)

[bxdxnn]

It helps to provide screenshots. Your PR doesn't describe what it changes.

This is a PoC implementation of matrix-org/matrix-spec-proposals#4274 so the current UI/behavior will change. I've linked this PR to a meta issue for better clarity.

Sounds like it may do the same as #31932 which has had feedback from Design which probably also apply here #31932 (review)

I'm not sure what that PR is about, and it doesn't touch the JS SDK so it doesn't seems to be the same.
Design is currently WIP for Web, Android/iOS is already done (I'm implementing it ATM), see element-hq/element-x-android#6519 (comment)