feat(ui): add database stats card on database page

Show table info (entry count, oldest/newest dates, total DB size) in a
new card on the database page, with auto-refresh and i18n support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

html/lang/en.json

@@ -92,7 +92,13 @@
     "dangerZone": "Danger Zone",
     "dangerWarning": "Warning: This action is irreversible!",
     "emptyAllTables": "Empty all sensor tables",
-    "emptyTablesNote": "Note: Configuration and timestamp tables will be preserved."
+    "emptyTablesNote": "Note: Configuration and timestamp tables will be preserved.",
+    "statsTitle": "Database Information",
+    "statsDbSize": "Total size:",
+    "statsTable": "Table",
+    "statsCount": "Entries",
+    "statsOldest": "Oldest",
+    "statsNewest": "Newest"
   },
   "logs": {
     "title": "The Log",

html/lang/fr.json

@@ -92,7 +92,13 @@
     "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."
+    "emptyTablesNote": "Note: Les tables de configuration et horodatage seront préservées.",
+    "statsTitle": "Informations sur la base",
+    "statsDbSize": "Taille totale:",
+    "statsTable": "Table",
+    "statsCount": "Entrées",
+    "statsOldest": "Plus ancienne",
+    "statsNewest": "Plus récente"
   },
   "logs": {
     "title": "Le journal",