feat: Hybride Passkey-Freigabe für Skipper und Crew

Skipper (nur Owner) und Crew (WRITE-Collaborators) können Logbuchseiten
optional per WebAuthn freigeben; klassische Unterschrift bleibt als Fallback.
Signatur ist an den Eintrags-Hash gebunden, Export in CSV/PDF angepasst.

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

client/src/i18n/locales/en.json

@@ -118,6 +118,17 @@
       "sign_hint": "Sign with finger, stylus, or mouse",
       "sign_clear": "Clear",
       "sign_export_image": "[Signature]",
+      "sign_with_passkey": "Sign with Passkey",
+      "sign_passkey_signing": "Requesting Passkey…",
+      "sign_passkey_signed": "Signed by {{username}}",
+      "sign_passkey_export": "Passkey: {{username}} ({{date}})",
+      "sign_passkey_clear": "Remove Passkey signature",
+      "sign_passkey_failed": "Passkey signing failed",
+      "sign_passkey_cancelled": "Passkey signing cancelled",
+      "sign_invalid": "Signature invalid — entry content changed",
+      "sign_classic_or_passkey": "Optional: sign classically below or use Passkey above",
+      "sign_crew_passkey_hint": "Write collaborators can sign with their Passkey",
+      "sign_offline_hint": "Passkey signing requires internet — classic signature works offline",
       "no_entries": "No logbook entries found for this yacht. Create your first travel day to begin!",
       "back_to_list": "Back to Journal List",
       "save": "Save Logbook Page",