feat: Web Push für Logbuch-Eigner bei Crew-Sync

Benachrichtigt Owner optional per VAPID/Web Push, wenn Collaborators
Änderungen synchronisieren — ohne Klartext-Inhalte, mit Opt-in in den
Einstellungen, Custom Service Worker und Deep-Link zum Logbuch.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/i18n/locales/en.json

@@ -334,6 +334,14 @@
       "tour_title": "App tour",
       "tour_desc": "Take a guided walkthrough of the main areas of the app again.",
       "tour_restart": "Restart tour",
+      "push_title": "Push notifications",
+      "push_desc": "As logbook owner you are notified when invited crew members sync changes. No logbook content is sent in plain text.",
+      "push_enable": "Notify on crew changes",
+      "push_active": "Push notifications are active on this device.",
+      "push_unsupported": "Push notifications are not supported in this browser.",
+      "push_denied_hint": "Notifications are blocked. Allow them in your browser or device settings.",
+      "push_ios_install_hint": "On iPhone/iPad: add the app to your Home Screen (iOS 16.4+) to use push notifications.",
+      "push_error": "Could not enable push notifications.",
       "backup_title": "Backup & restore",
       "backup_desc": "Full encrypted backup of this logbook (entries, photos, GPS tracks, crew, vessel). Protected with a backup passphrase — restore on this or a new account.",
       "backup_export_title": "Create backup",