Add MicroQuickJS code executor (#180)

* Add MicroQuickJS code executor

Creates microquickjs.html - a sandboxed JavaScript executor using
Fabrice Bellard's MicroQuickJS engine compiled to WebAssembly.

Features:
- Minimal ES5-like JavaScript sandbox
- Memory and time limits via WASM isolation
- No console.log (returns last expression value)
- Example snippets adapted for MicroQuickJS dialect
- URL hash sharing for code snippets

Includes mquickjs.js and mquickjs.wasm from research repository.

* Fix button selector in mobile media query

Make button width: 100% only apply to buttons within .button-group,
not all buttons (like example buttons).

* Increase textarea font-size to 16px to prevent mobile Safari zoom

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Co-authored-by: Claude <noreply@anthropic.com>

Author: simonw