fix: accessibility heading semantics [WPB-14776]

[Garzas]

https://wearezeta.atlassian.net/browse/WPB-14776

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PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

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What's new in this PR?

• Marked visual screen titles and section labels as accessibility headings across login/onboarding, conversations, archive, settings, user profile, conversation details, and call-related screens.
• Added heading semantics to shared UI components such as section headers and empty-state titles so reused views expose the correct structure to TalkBack.
• Added Compose instrumentation coverage to verify key heading components expose SemanticsProperties.Heading.

[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

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.77%. Comparing base (1f84f90) to head (dab52c4).

Files with missing lines	Patch %	Lines
...id/ui/newauthentication/login/NewLoginContainer.kt	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4972      +/-   ##
===========================================
- Coverage    48.77%   48.77%   -0.01%     
===========================================
  Files          644      645       +1     
  Lines        23151    23152       +1     
  Branches      3546     3546              
===========================================
  Hits         11292    11292              
- Misses       10811    10812       +1     
  Partials      1048     1048              

Files with missing lines	Coverage Δ
...ettings/devices/EndToEndIdentityCertificateItem.kt	0.00% <ø> (ø)
...id/ui/newauthentication/login/NewLoginContainer.kt	0.00% <0.00%> (ø)

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