Add database cleanup feature to empty all sensor tables

Added a "Danger Zone" section on the database page that allows users to empty all sensor data tables while preserving configuration and timestamp tables. The feature includes:

- New Python script (sqlite/empty_sensor_tables.py) to safely empty sensor tables
- Backend endpoint in launcher.php (empty_sensor_tables)
- Frontend UI with red warning card and confirmation dialog
- Detailed feedback showing deleted record counts per table
- i18n support for French and English

Tables emptied: data_NPM, data_NPM_5channels, data_BME280, data_envea, data_WIND, data_MPPT, data_NOISE, modem_status
Tables preserved: timestamp_table, config_table, envea_sondes_table, config_scripts_table

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

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

html/lang/en.json

@@ -88,7 +88,11 @@
     "timestampTable": "Timestamp Table",
     "downloadData": "Download Data",
     "startDate": "Start date:",
-    "endDate": "End date:"
+    "endDate": "End date:",
+    "dangerZone": "Danger Zone",
+    "dangerWarning": "Warning: This action is irreversible!",
+    "emptyAllTables": "Empty all sensor tables",
+    "emptyTablesNote": "Note: Configuration and timestamp tables will be preserved."
   },
   "logs": {
     "title": "The Log",

html/lang/fr.json

@@ -88,7 +88,11 @@
     "timestampTable": "Timestamp Table",
     "downloadData": "Télécharger les données",
     "startDate": "Date de début:",
-    "endDate": "Date de fin:"
+    "endDate": "Date de fin:",
+    "dangerZone": "Zone dangereuse",
+    "dangerWarning": "Attention: Cette action est irréversible!",
+    "emptyAllTables": "Vider toutes les tables de capteurs",
+    "emptyTablesNote": "Note: Les tables de configuration et horodatage seront préservées."
   },
   "logs": {
     "title": "Le journal",