fix(streamdown): improve mermaid diagram sizing and responsiveness

[ayhansipahi]

Description

Mermaid SVG diagrams could overflow their container, especially on smaller viewports. This PR constrains diagrams to their container width and improves layout consistency.

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

Changes Made

• Constrain mermaid SVG diagrams to container width using max-w-full, h-auto, and w-full CSS classes
• Add min-w-0 to chart container for proper flexbox shrinking
• Increase minimum container height from min-h-28 to min-h-48 for better initial layout
• Align non-fullscreen diagrams to items-start instead of no alignment

Testing

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

Test Coverage

Added 2 new tests:

• should apply responsive sizing classes to chart container — verifies w-full, min-w-0, items-start in normal mode
• should apply fullscreen alignment to chart container — verifies items-center, size-full in fullscreen mode

All 17 mermaid component tests pass.

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

[vercel]

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