Fix bizard-skill.zip download link resolving to wrong path on sub-pages

[Copilot]

The bizard-skill.zip download link was broken on any page below the site root (e.g. Distribution/Density.html), resolving to /bizard-skill.zip instead of /Bizard/bizard-skill.zip.

Root Cause

The JS path calculation subtracted only 2 from parts.length — accounting for the leading '' and the site root segment — but forgot to subtract the filename itself:

// Before: off by one — counts the filename as a directory to traverse
var segmentsAfterRoot = parts.length - 2;

// After: correctly excludes '', site-root, and the filename
var segmentsAfterRoot = Math.max(0, parts.length - 3);

For /Bizard/Distribution/Density.html (parts.length = 4):

• Before: 4 - 2 = 2 → ../../bizard-skill.zip → /bizard-skill.zip ❌
• After: max(0, 4 - 3) = 1 → ../bizard-skill.zip → /Bizard/bizard-skill.zip ✅

Changes

• skill-button.lua — fixes the button injected into every tutorial page
• Skills.qmd / Skills.zh.qmd — fixes the same logic on the Skills landing pages