fix: simplified layout and fixed chat title bar button

[lorenzo-ranciaffi]

Pull Request Description

Fixes #5682

What does this PR change?

This PR simplifies the chat title bar hierarchy by removing all content size fitter and managing everything by layouts. On top of that, few tweaks were needed to restore the button in order to interact with entity of the conversation (community or user)

Test Instructions

Test Steps

1. Verify that the repro steps in the linked issue are now fixed
2. Check all possible interaction with the chat title bar (context menus, voice calls, etc...) and that it always behaves as normal
3. Check that the title bar behaves normally even with long user/community names

Quality Checklist

• Changes have been tested locally
• Documentation has been updated (if required)
• Performance impact has been considered
• For SDK features: Test scene is included

Code Review Reference

Please review our Code Review Standards before submitting.

[github-actions]

Windows and Mac build successful in Unity Cloud! You can find a link to the downloadable artifact below.

Name	Link
Commit	448f0a1
Logs	https://github.com/decentraland/unity-explorer/actions/runs/18345621842
Download Windows	https://github.com/decentraland/unity-explorer/suites/47175958582/artifacts/4215529950
Download Windows S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/fix/5682-chat-titlebar-button/pr-14744-448f0a1/Decentraland_windows64.zip
Download Mac	https://github.com/decentraland/unity-explorer/suites/47175958582/artifacts/4215305103
Download Mac S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/fix/5682-chat-titlebar-button/pr-14744-448f0a1/Decentraland_macos.zip
Built on	2025-10-08T13:51:25Z

[pravusjif]

QA Review has more weight in this one

[Ludmilafantaniella]

✅ Approve — Verified on Windows and Mac.
The main issue is fixed and the chat title bar works as expected.
I’ll create a separate ticket for the button state (hover color not persisting on click) so we can handle that improvement separately.