feat: Improve Mermaid diagram viewing experience

[flex-yj-kim]

Summary

This PR improves the end-to-end Mermaid diagram viewing experience, especially for large architecture diagrams that were difficult to inspect in the default plan width.

What changed

• Introduced a stronger Mermaid viewing flow centered around:
  • expanded fullscreen diagram mode
  • practical zoom controls (zoom in/out, fit-to-view)
  • quick source/diagram toggling
• Updated fit behavior to re-center and fit diagrams to the current viewport/container, rather than only resetting zoom.
• Improved wheel zoom behavior in expanded view so navigation feels consistent during review.
• Rendered the expanded Mermaid overlay at the document level to avoid layout-context clipping.
• Improved control clarity and readability (icon tuning, control grouping, zoom badge).
• Increased viewer widths in both plan view and plan-diff view to better support wide diagrams.
• Ensured the new fullscreen Mermaid viewing flow works consistently in Safari by stabilizing overlay and viewport behavior.

Screenshots