restructure: move icons into libraries/, vendor files out of git

- lib-icons/ -> libraries/icons/ (C library icon SVGs stay with their libs)
- libraries/highlight/ -> vendor/highlight/ (not committed, downloaded)
- lib/mathjax/ -> vendor/mathjax/ (not committed, downloaded)
- vendor/download.sh fetches highlight.js v11.10.0 and MathJax v3.2.2
- index.html paths updated to vendor/
- .gitignore ignores vendor/* (except download.sh)
- Removes ~1.2MB of binary font files from git tracking

.gitignore

@@ -0,0 +1,4 @@
+# Vendor assets (downloaded at deploy time)
+vendor/highlight/*
+vendor/mathjax/*
+!vendor/download.sh