KaTeX renders square roots using a 400em-wide SVG that relies on CSS
`overflow: hidden` to clip to the correct length. Without the KaTeX
stylesheet, this SVG extends across the entire page.
Changes:
- Add KaTeX CDN stylesheet link to base.html for fonts and layout
- Add .katex-display local margin/overflow overrides
- Add .hide-tail svg max-width fallback for file:// CORS blocking
The CDN provides full KaTeX styling when accessible. The SVG constraint
serves as a fallback when CDN is blocked (e.g., local file:// access).
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