Site redesign, new content, blog gen, E2E tests, and build improvements

New blog posts (monopam-madness, open-source-and-ai, weeknotes-2026-10),
notebook showcase with card layout and screenshots, Atom feed generator,
foundations notebook fixes, ONNX test improvements, widget interaction
tests, deploy script updates for oxcaml switch, and .gitignore for
build artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>