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>

html/lang/en.json

@@ -0,0 +1,53 @@
+{
+  "common": {
+    "getData": "Get Data",
+    "loading": "Loading...",
+    "error": "Error",
+    "startRecording": "Start recording",
+    "stopRecording": "Stop recording"
+  },
+  "sensors": {
+    "title": "Measurement Sensors",
+    "description": "Your NebuleAir sensor is equipped with one or more probes that measure environmental variables. Measurements are automatic, but you can verify their operation here.",
+    "npm": {
+      "title": "NextPM",
+      "description": "Particulate matter sensor.",
+      "headerUart": "UART Port"
+    },
+    "bme280": {
+      "title": "BME280 Temp/Humidity Sensor",
+      "description": "Temperature and humidity sensor on I2C port.",
+      "headerI2c": "I2C Port",
+      "temp": "Temperature",
+      "hum": "Humidity",
+      "press": "Pressure"
+    },
+    "noise": {
+      "title": "Decibel Meter",
+      "description": "Noise sensor on I2C port.",
+      "headerI2c": "I2C Port"
+    },
+    "envea": {
+      "title": "Envea Probe",
+      "description": "Gas sensor."
+    }
+  },
+  "wifi": {
+    "title": "WIFI Connection",
+    "description": "WIFI connection is not mandatory but it allows you to perform updates and enable remote control.",
+    "status": "Status",
+    "connected": "Connected",
+    "hotspot": "Hotspot",
+    "disconnected": "Disconnected",
+    "scan": "Scan",
+    "connect": "Connect",
+    "enterPassword": "Enter password for"
+  },
+  "admin": {
+    "title": "Administration",
+    "parameters": "Parameters (config)",
+    "deviceName": "Device Name",
+    "deviceID": "Device ID",
+    "modemVersion": "Modem Version"
+  }
+}