feat(settings): add screensaver toggle in Language tab (default off)

Toggle appears in the Language settings tab, below the language selector.
Default: disabled. When disabled, initInactivityWatcher() exits early so
the screensaver never activates. i18n added for it/en/de.

translations/de.json

@@ -595,6 +595,9 @@
             "label": "🌐 Sprache",
             "restart_notice": "Die Seite wird neu geladen, um die neue Sprache anzuwenden."
         },
+        "screensaver": {
+            "label": "Bildschirmschoner aktivieren"
+        },
         "scale": {
             "title": "⚖️ Smart-Waage",
             "hint": "Verbinde eine Bluetooth-Waage über das Android-Gateway, um das Gewicht automatisch auszulesen.",

translations/en.json

@@ -594,6 +594,9 @@
             "label": "🌐 Language",
             "restart_notice": "The page will reload to apply the new language."
         },
+        "screensaver": {
+            "label": "Enable screensaver"
+        },
         "scale": {
             "title": "⚖️ Smart Scale",
             "hint": "Connect a Bluetooth scale via the Android gateway to automatically read weight.",

translations/it.json

@@ -594,6 +594,9 @@
             "label": "🌐 Lingua",
             "restart_notice": "La pagina verrà ricaricata per applicare la nuova lingua."
         },
+        "screensaver": {
+            "label": "Attiva salvaschermo"
+        },
         "scale": {
             "title": "⚖️ Bilancia Smart",
             "hint": "Collega una bilancia Bluetooth tramite il gateway Android per leggere il peso automaticamente.",