Build QuickJS code executor with live results (#166)

* Add quickjs.html - JavaScript code executor using QuickJS WebAssembly

A UI that lets users enter JavaScript code in a textarea and execute it
using quickjs-emscripten. Features:

- Sandboxed execution via QuickJS running in WebAssembly
- Console.log/warn/error/info support
- URL hash state persistence (code saved as #url-encoded-javascript)
- Auto-execute when loading page with hash
- Keyboard shortcut (Ctrl/Cmd + Enter) to run code
- Tab key support for indentation
- Copy output to clipboard
- Execution time display

* Fix QuickJS loading and add Playwright tests

Changes to quickjs.html:
- Switch from esm.sh to cdn.jsdelivr.net (fixes MIME type errors)
- Use IIFE global build instead of ES modules (better compatibility)
- Fix version from 0.31.1 (doesn't exist) to 0.31.0

Add test infrastructure:
- tests/conftest.py: Improved server fixture with proper startup waiting
- tests/test_quickjs.py: Comprehensive Playwright tests for QuickJS executor
  - Tests skip gracefully when CDN is unreachable (isolated environments)
  - Covers page load, initialization, code execution, URL hash state, etc.

* Add interactive examples with pill-style buttons

- Added 10 clickable example snippets: Hello World, Factorial, Fibonacci,
  Array Methods, Objects & JSON, Prime Numbers, String Fun, FizzBuzz,
  Sorting, and Date & Time
- Examples load into textarea and auto-execute when clicked
- Pill-style buttons with hover effects for clean UI
- Removed placeholder text in favor of example buttons

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

Author: simonw

quickjs.docs.md

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+Execute JavaScript code in a sandboxed QuickJS environment running via WebAssembly. Code is automatically saved in the URL hash for easy sharing.