feat(i18n): add language selector UI to docs page

Added interactive language switcher dropdown to make translations accessible:

UI Changes:
- Added language selector dropdown to docs.html header
- Flag emojis + language names (🇬🇧 English, 🇩🇪 Deutsch, 🇫🇷 Français)
- Positioned next to search button in page header

Functionality:
- Detect language from URL param, localStorage, or default to English
- Save language choice to localStorage for persistence
- Auto-reload document when language changes
- Update URL with ?lang= parameter
- Preserves selected document when switching languages

Implementation:
- Enhanced detectLanguage() to check URL > localStorage > i18n > default
- Added initLanguageSelector() IIFE to wire up dropdown
- Dropdown reflects current language on page load

User Experience:
- One-click language switching
- Language persists across page reloads
- Seamless document reload in new language
- URL updates to reflect language choice

🌐 Generated with Claude Code

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

public/docs.html

@@ -495,12 +495,25 @@
           <h1 class="text-3xl font-bold text-gray-900">Framework Documentation</h1>
           <p class="text-gray-600 mt-2">Technical specifications, guides, and reference materials</p>
         </div>
-        <button type="button" id="open-search-modal-btn" class="px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition flex items-center gap-2" aria-label="Open search">
-          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/>
-          </svg>
-          <span>Search</span>
-        </button>
+        <div class="flex items-center gap-3">
+          <!-- Language Selector -->
+          <div class="relative">
+            <select id="language-selector"
+                    class="px-4 py-2 bg-white border border-gray-300 rounded-lg hover:border-gray-400 transition text-sm font-medium text-gray-700 cursor-pointer focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-transparent"
+                    aria-label="Select language">
+              <option value="en">🇬🇧 English</option>
+              <option value="de">🇩🇪 Deutsch</option>
+              <option value="fr">🇫🇷 Français</option>
+            </select>
+          </div>
+          <!-- Search Button -->
+          <button type="button" id="open-search-modal-btn" class="px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition flex items-center gap-2" aria-label="Open search">
+            <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/>
+            </svg>
+            <span>Search</span>
+          </button>
+        </div>
       </div>
     </div>
   </div>

public/js/docs-app.js

@@ -8,14 +8,28 @@ if (typeof DocumentCards !== 'undefined') {
   documentCards = new DocumentCards('document-content');
 }
 
-// Detect language from i18n system
+// Detect language from URL, localStorage, or i18n system
 function detectLanguage() {
+  // Priority 1: URL parameter
+  const urlParams = new URLSearchParams(window.location.search);
+  const urlLang = urlParams.get('lang');
+  if (urlLang) {
+    return urlLang;
+  }
+
+  // Priority 2: localStorage
+  const storedLang = localStorage.getItem('tractatus_language');
+  if (storedLang) {
+    return storedLang;
+  }
+
+  // Priority 3: i18n system
   if (window.I18n && window.I18n.currentLang) {
     return window.I18n.currentLang;
   }
-  // Fallback: Check URL parameter
-  const urlParams = new URLSearchParams(window.location.search);
-  return urlParams.get('lang') || 'en';
+
+  // Default: English
+  return 'en';
 }
 
 // Update URL with language parameter
@@ -627,6 +641,40 @@ function closeToCModal() {
 // Initialize
 loadDocuments();
 
+// Initialize language selector
+(function initLanguageSelector() {
+  const selector = document.getElementById('language-selector');
+  if (!selector) return;
+
+  // Set initial value from current language
+  const initialLang = detectLanguage();
+  selector.value = initialLang;
+  currentLanguage = initialLang;
+
+  // Handle language change
+  selector.addEventListener('change', (e) => {
+    const newLang = e.target.value;
+
+    // Save to localStorage
+    localStorage.setItem('tractatus_language', newLang);
+
+    // Update current language
+    currentLanguage = newLang;
+
+    // Reload current document in new language
+    if (currentDocument) {
+      loadDocument(currentDocument.slug, newLang);
+    } else {
+      // If no document loaded yet, just update URL
+      const urlParams = new URLSearchParams(window.location.search);
+      const currentDoc = urlParams.get('doc');
+      if (currentDoc) {
+        loadDocument(currentDoc, newLang);
+      }
+    }
+  });
+})();
+
 // Add ESC key listener for closing modal
 document.addEventListener('keydown', (e) => {
   if (e.key === 'Escape') {