The mermaid-init script was throwing 'Uncaught TypeError: can't access
property addEventListener, document.getElementById(...) is null' because
it tried to attach click handlers to theme toggle buttons (ayu, navy, coal,
light, rust) that may not exist on all pages.
Fixed by:
- Wrapping theme button event listener registration in null checks
- Using DOMContentLoaded fallback when document is still loading
- Keeping mermaid.initialize() call outside the check since it needs to
run early with startOnLoad: true
