Implement lightweight offline i18n system with French/English support

**Core System:**
- Add i18n.js translation library with data-attribute support
- Create translation files (fr.json, en.json) with offline support
- Store language preference in SQLite config_table
- Add backend endpoints for get/set language

**UI Features:**
- Add language switcher dropdown to topbar (🇫🇷 FR / 🇬🇧 EN)
- Auto-sync language selection across all pages
- Support for static HTML and dynamically created elements

**Implementation:**
- Migrate sensors.html as working example
- Add data-i18n attributes to all UI elements
- Support for buttons, inputs, and dynamic content
- Comprehensive README documentation in html/lang/

**Technical Details:**
- Works completely offline (local JSON files)
- No external dependencies
- Database-backed user preference
- Event-based language change notifications
- Automatic translation on page load

Next steps: Gradually migrate other pages (admin, wifi, index, etc.)

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

sqlite/set_config.py

@@ -50,7 +50,8 @@ config_entries = [
     ("BME280", "0", "bool"),
     ("MPPT", "0", "bool"),
     ("NOISE", "0", "bool"),
-    ("modem_version", "XXX", "str")
+    ("modem_version", "XXX", "str"),
+    ("language", "fr", "str")
 ]
 
 for key, value, value_type in config_entries: