Announce checked/unchecked state for user rows in invite screen

[bmarty]

Summary

• Replaces .clickable(role = Role.Checkbox) with .toggleable(value = checked, role = Role.Checkbox) in CheckableUserRow so TalkBack announces the current checked state and state changes (fixes A11y: [Invite people] Screen reader does not announce updated state when... #6407)
• Sets Checkbox.onCheckedChange = null to make the inner checkbox a purely visual indicator, avoiding duplicate TalkBack focus on the same control

Test plan

• Open the Invite People screen with TalkBack
• Focus a user row — verify TalkBack announces "checked" or "unchecked"
• Tap to toggle — verify TalkBack announces the state change

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

📱 Scan the QR code below to install the build (arm64 only) for this PR.

If you can't scan the QR code you can install the build via this link: https://i.diawi.com/EfxdVi

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.54%. Comparing base (58b3830) to head (26e4e89).
⚠️ Report is 100 commits behind head on develop.

Files with missing lines	Patch %	Lines
...libraries/matrix/ui/components/CheckableUserRow.kt	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7136   +/-   ##
========================================
  Coverage    80.54%   80.54%           
========================================
  Files         2689     2689           
  Lines        76433    76433           
  Branches     10291    10291           
========================================
  Hits         61566    61566           
  Misses       10933    10933           
  Partials      3934     3934           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[bmarty]

I confirm that this still works as expected with this change, hence the previous code was not as the issue description. The check state was read out loud.