feat(i18n): enhance browser language detection with clear priority logging

SUMMARY:
Enhanced i18n language detection to clearly show priority order and
provide better logging for debugging user language preferences.

PRIORITY ORDER:
1. User's manual selection (localStorage) - HIGHEST
   - Clicking language flags saves preference here
   - Allows users to override browser language

2. Browser's language setting (navigator.language) - AUTOMATIC
   - Detects from browser preferences
   - Only applies if no user override exists

3. Default to English (fallback)
   - Used when browser language not supported

CHANGES:

1. Enhanced detectLanguage():
   - Added clear comments documenting priority order
   - Added console.log for each detection path
   - Shows which source determined the language
   - Logs browser language even when not supported

2. Enhanced setLanguage():
   - Added log when user manually selects language
   - Clarifies that preference overrides browser detection
   - Shows that preference persists across pages

BENEFITS:
✓ Users see automatic language detection from browser
✓ Users can override via flag clicks (persists via localStorage)
✓ Clear logging helps debug language selection issues
✓ Developers can see exactly how language was determined

EXAMPLE LOGS:
- Browser detection: '[i18n] Language detected from browser: de (from de-DE)'
- User override: '[i18n] User manually selected language: fr (saved to localStorage)'
- Fallback: '[i18n] Language defaulted to: en (browser language 'ja-JP' not supported)'

🤖 Generated with Claude Code (https://claude.com/claude-code)

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

public/js/i18n-simple.js

@@ -25,19 +25,27 @@ const I18n = {
   },
   
   detectLanguage() {
-    // Check localStorage first
+    // Priority order:
+    // 1. User's manual selection (localStorage) - allows override via flag clicks
+    // 2. Browser's language setting (automatic detection)
+    // 3. Default to English (fallback)
+
+    // 1. Check localStorage first (user override)
     const saved = localStorage.getItem('tractatus-lang');
     if (saved && this.supportedLanguages.includes(saved)) {
+      console.log(`[i18n] Language detected from user preference: ${saved}`);
       return saved;
     }
-    
-    // Check browser language
+
+    // 2. Check browser language (automatic detection)
     const browserLang = (navigator.language || navigator.userLanguage).split('-')[0];
     if (this.supportedLanguages.includes(browserLang)) {
+      console.log(`[i18n] Language detected from browser: ${browserLang} (from ${navigator.language})`);
       return browserLang;
     }
-    
-    // Default to English
+
+    // 3. Default to English
+    console.log(`[i18n] Language defaulted to: en (browser language '${navigator.language}' not supported)`);
     return 'en';
   },
   
@@ -160,22 +168,23 @@ const I18n = {
       console.error(`[i18n] Unsupported language: ${lang}`);
       return;
     }
-    
-    // Save preference
+
+    // Save preference (overrides browser language detection)
     localStorage.setItem('tractatus-lang', lang);
-    
+    console.log(`[i18n] User manually selected language: ${lang} (saved to localStorage)`);
+
     // Update current language
     this.currentLang = lang;
-    
+
     // Reload translations
     await this.loadTranslations(lang);
-    
+
     // Reapply to page
     this.applyTranslations();
-    
+
     // Update selector
     this.updateLanguageSelector();
-    
+
     // Update HTML lang attribute
     document.documentElement.lang = lang;
 
@@ -184,7 +193,7 @@ const I18n = {
       detail: { language: lang }
     }));
 
-    console.log(`[i18n] Language changed to: ${lang}`);
+    console.log(`[i18n] Language changed to: ${lang} (will persist across pages)`);
   },
   
   updateLanguageSelector() {