Fix mermaid fullscreen overlay accessibility and add stable data-streamdown attribute

[aradhyacp]

Description

This PR fixes accessibility and consistency issues in the mermaid fullscreen overlay by aligning it with the existing table fullscreen implementation. It also introduces a stable attribute for external styling and tooling.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Fixes #506
Closes #506
Related to #506

Changes Made

• Added data-streamdown="mermaid-fullscreen" to the fullscreen overlay for stable CSS targeting
• Updated overlay semantics from role="button" to role="dialog"
• Added aria-modal="true" to improve screen reader modal behavior
• Aligned mermaid fullscreen overlay behavior with table fullscreen implementation

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

N/A (UI-only accessibility and structural change)

Screenshots/Demos

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

This change brings parity between mermaid and table fullscreen overlays and improves accessibility compliance for modal behavior in screen readers.

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