Fix print CSS partially applied due to external stylesheet timing

[patakuti]

Closes #59

Summary

• printPage() now waits for all <link rel="stylesheet"> elements to finish loading before calling window.print()
• Cross-origin stylesheets (e.g. KaTeX CDN) that block .sheet access via CORS are skipped gracefully
• A 3-second timeout ensures the print dialog is never blocked indefinitely if a stylesheet fails to load

Root Cause

On page load, switchTheme() is called with the theme from localStorage, which changes href on the theme <link> element and triggers a new HTTP request. Calling window.print() before this request completes results in a print preview without theme CSS applied.

Test plan

• Open a markdown file, confirm a non-default theme is saved in localStorage
• Click Print immediately after page load — verify theme CSS (colors, fonts) is correctly applied in print preview
• Click Print after page is fully loaded — verify normal behavior unchanged

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