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>

.env.example

@@ -4,4 +4,10 @@ OpenWeatherMapAPIKey=<owm_api_key>
 # For local dev: localhost and http://localhost
 # For production: e.g. kapteins-daagbok.eu and https://kapteins-daagbok.eu
 RP_ID=localhost
-ORIGIN=http://localhost
+ORIGIN=http://localhost
+
+# Web Push (VAPID) — generate with: npx web-push generate-vapid-keys
+# Public key may also be set on the client as VITE_VAPID_PUBLIC_KEY
+VAPID_PUBLIC_KEY=
+VAPID_PRIVATE_KEY=
+VAPID_SUBJECT=mailto:support@kapteins-daagbok.eu