Fix Pyodide sandbox on Windows

[jlowin]

The Pyodide sandbox fails completely on Windows due to three independent issues reported in #393. All three are fixed here, and the Windows skip is removed from CI so they're tested going forward.

CommonJS detection: Deno 2.x needs --unstable-detect-cjs to load Pyodide's npm package, which uses CommonJS modules.

WASM OOM: Pyodide's ~20MB WASM binary exceeds Deno's default V8 heap on Windows. Setting --max-old-space-size=4096 prevents the crash during initialization.

Backslash paths: runner.js uses walk() paths directly for Pyodide's VFS, which only understands forward slashes. On Windows, entry.path contains backslashes, so prefab_ui\components\button.py gets mounted at the wrong path.

// Before:
const relPath = entry.path.slice(prefabSrc.length);
// After:
const relPath = entry.path.slice(prefabSrc.length).replaceAll("\\", "/");

CI now installs Deno on all platforms so the sandbox tests run on Windows.

Closes #393