feat: add HIG Playground - interactive iOS component demos

Add a new HIG Playground section with 15 interactive iOS component demos
rendered inside a CSS-only iPhone mockup frame. Each demo includes a
controls panel, live SwiftUI code preview, and HIG Do/Don't guidelines.

Components: Buttons, Toggles, Segmented Controls, Progress Indicators,
Sliders, Navigation Bars, Tab Bars, Lists, Alerts, Action Sheets,
Sheets, Context Menus, Text Fields, Date Pickers, Scroll Views.

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

Author: M1zz

site/en/index.html

@@ -76,6 +76,7 @@
     <div class="nav">
       <a href="https://github.com/M1zz/HIGLab/blob/main/GETTING_STARTED.md" data-i18n="nav_start">🌱 Get Started</a>
       <a href="roadmap.html" data-i18n="nav_roadmap">🗺️ Roadmap</a>
+      <a href="../hig-playground/" data-i18n="nav_playground">🎨 Playground</a>
       <a href="#ai-reference">🤖 AI Reference</a>
       <a href="https://github.com/M1zz/HIGLab">GitHub</a>
       <a href="https://youtube.com/@leeo25" data-i18n="nav_channel">YouTube</a>