i18n: Du-Ansprache und Einladungstexte in Locales auslagern

Deutsche UI-Texte und Beta-Flyer auf informelles Deutsch umstellen;
hardcodierte Strings aus InvitationAcceptance in de/en.json verschieben.

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

client/src/i18n/locales/en.json

@@ -427,6 +427,36 @@
       "cta_register": "Create account",
       "back_to_login": "Back to login"
     },
+    "invitation": {
+      "error_invalid_key": "The invitation link is cryptographically invalid (corrupted key).",
+      "error_missing_key": "The invitation link is missing the decryption key (#key=...). Please use the complete link from the owner.",
+      "error_expired": "This invitation link has expired (valid for 48 hours only).",
+      "error_invalid_token": "Failed to verify invitation token.",
+      "error_load_failed": "Invitation details could not be retrieved.",
+      "error_incomplete_session": "Incomplete session — please log in again (user ID missing).",
+      "error_accept_failed": "Acceptance failed.",
+      "error_login_failed": "Passkey authentication failed.",
+      "error_username_missing": "Could not determine username — please try logging in again.",
+      "error_register_failed": "Registration failed.",
+      "loading_joining": "Joining...",
+      "loading_checking": "Checking Invitation...",
+      "loading_unlocking": "Unlocking logbook and syncing data...",
+      "loading_retrieving_key": "Retrieving encryption key...",
+      "error_title": "Invitation Error",
+      "back_to_start": "Back to Dashboard",
+      "title": "Logbook Invitation",
+      "invited_by": "INVITED BY",
+      "vessel_logbook": "VESSEL / LOGBOOK",
+      "signed_in_preparing": "Signed in as {{username}}. Preparing to join...",
+      "join_again": "Join again",
+      "login_or_register_hint": "Sign in or register an account to join this logbook.",
+      "or_sign_up": "OR SIGN UP",
+      "register_crew_account": "Register New Crew Account",
+      "username_label": "Username",
+      "create_passkey": "Create Passkey",
+      "switch_language_en": "English",
+      "switch_language_de": "Deutsch"
+    },
     "stats": {
       "title": "Statistics",
       "subtitle": "Routes, consumption and propulsion at a glance",