follow-up: a11y + long-session hardening for Mermaid zoom modal

[AmirTlinov]

Context

Initial inspect/zoom UX for Mermaid diagrams is being introduced in #359 (implementation candidate in #358).

Problem

The modal interaction should be hardened for accessibility and long-lived preview sessions.

Goal

Ship production-level UX hardening for the Mermaid inspect modal.

Scope

In scope

• Keyboard accessibility:
  • Esc closes modal
  • Focus management and focus return
  • Keyboard operation for zoom controls
• A11y semantics:
  • role="dialog", aria-modal, labels for controls
• Stability:
  • Stress test repeated open/close cycles and live-update rerenders
  • Ensure no leaked handlers / duplicate DOM nodes over long sessions

Out of scope

• Re-design of visual style.
• New external UI frameworks.

Acceptance criteria

• Modal can be fully operated with keyboard only.
• Focus trap/restore behavior is correct.
• ARIA attributes are present and valid.
• Repeated open/close and rerender cycles do not create duplicate nodes/listeners.
• No regression in preview performance.

Verification

• Manual keyboard test matrix.
• Browser/runtime smoke checks for DOM/listener idempotency.
• Long-session test (>10 min with frequent rerenders).

References

• Main feature issue: feat: Mermaid diagram inspect/zoom UX in preview #359
• Current implementation PR: feat: add mermaid zoom modal controls #358