Add Japanese version (/ja) and language switcher with globe icon

- Add /ja/index.html with full Japanese translation (code snippets
  stay in English), sharing CSS/JS via relative paths
- Add globe icon language switcher to header (JA/EN toggle)
- Add Noto Sans JP to Google Fonts and CSS font stack for proper
  Japanese rendering
- Keep language switcher visible on mobile (GitHub link hidden)
- Update hero catchcopy to AI agent messaging

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mokuwaki

index.html

@@ -9,7 +9,7 @@
     <!-- Fonts -->
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-    <link href="https://fonts.googleapis.com/css2?family=Fira+Code:wght@400;500;600&family=Inter:wght@400;500;600;700;800&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=Fira+Code:wght@400;500;600&family=Inter:wght@400;500;600;700;800&family=Noto+Sans+JP:wght@400;500;600;700;800&display=swap" rel="stylesheet">
 
     <!-- Styles -->
     <link rel="stylesheet" href="style.css">
@@ -30,6 +30,10 @@
                 <img src="./assets/nipry-logo.svg" alt="nipry logo" class="logo-img">
             </a>
             <div class="nav nav-right">
+                <a href="/ja/" class="nav-link lang-switch" aria-label="日本語に切り替え">
+                    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lang-icon"><circle cx="12" cy="12" r="10"></circle><path d="M12 2a14.5 14.5 0 0 0 0 20 14.5 14.5 0 0 0 0-20"></path><path d="M2 12h20"></path></svg>
+                    JA
+                </a>
                 <a href="https://github.com/co-r-e/nipry" target="_blank" rel="noopener noreferrer" class="nav-link github-link">
                     GitHub
                     <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="external-icon"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"></path><polyline points="15 3 21 3 21 9"></polyline><line x1="10" y1="14" x2="21" y2="3"></line></svg>
@@ -45,7 +49,7 @@
             <div class="hero-overlay"></div>
             <div class="container hero-container">
                 <div class="hero-content">
-                    <h1 class="hero-title"><span class="highlight">Stay in Flow.</span> Ship Slides from Your Terminal.</h1>
+                    <h1 class="hero-title"><span class="highlight">Your AI Agent Builds the Deck.</span> You Never Leave the Terminal.</h1>
                     <p class="hero-subtitle">
                         You already use Claude Code, Codex, Gemini CLI, or Cursor for coding.
                         Now ask the same agent to build your slide deck — without leaving your terminal.