feat(register): redesign register page to match spec

Replaces the minimal login-style form with the full spec design:
hero section (eyebrow, headline, subtext), three labelled form sections,
2-column name grid, confirm-password field with client-side match hints,
password strength indicator, notification checkbox card, loading state on
submit, and "already have an account?" footer link.

Backend: adds notifyOnMention to RegisterRequest and wires both
notifyOnMention and notifyOnReply via updateNotificationPreferences on
invite redemption.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -658,6 +658,22 @@
 	"register_invalid_code": "Ungültiger Einladungslink",
 	"register_invalid_code_desc": "Dieser Einladungslink ist nicht gültig, wurde bereits verwendet oder ist abgelaufen. Bitte wende dich an den Administrator.",
 	"register_success": "Dein Konto wurde erfolgreich erstellt. Du kannst dich jetzt anmelden.",
+	"register_eyebrow": "Ein Familienprojekt",
+	"register_hero_headline": "Schön, dass du da bist.",
+	"register_hero_subtext": "Gemeinsam bewahren wir Briefe und Dokumente – für die Familie, jetzt und in Zukunft.",
+	"register_section_about": "Über dich",
+	"register_section_account": "Konto",
+	"register_section_notifications": "Benachrichtigungen",
+	"register_label_password_confirm": "Passwort bestätigen",
+	"register_pw_hint": "Mindestens 8 Zeichen.",
+	"register_pw_ok": "Mindestens 8 Zeichen. ✓",
+	"register_pw_match_ok": "Passwörter stimmen überein.",
+	"register_pw_match_no": "Die beiden Passwörter stimmen noch nicht überein.",
+	"register_notify_title": "Benachrichtige mich,",
+	"register_notify_desc": "wenn jemand mich in einem Kommentar erwähnt oder mir auf einen Kommentar antwortet.",
+	"register_btn_loading": "Wird erstellt …",
+	"register_already_have_account": "Du hast bereits ein Konto?",
+	"register_sign_in": "Anmelden",
 	"login_registered_success": "Dein Konto wurde erfolgreich erstellt. Melde dich jetzt an.",
 	"admin_tab_invites": "Einladungen",
 	"admin_invites_list_title": "Einladungen",

frontend/messages/en.json

@@ -658,6 +658,22 @@
 	"register_invalid_code": "Invalid invite link",
 	"register_invalid_code_desc": "This invite link is not valid, has already been used, or has expired. Please contact the administrator.",
 	"register_success": "Your account has been created. You can now sign in.",
+	"register_eyebrow": "A family project",
+	"register_hero_headline": "Glad you're here.",
+	"register_hero_subtext": "Together we preserve letters and documents – for the family, now and in the future.",
+	"register_section_about": "About you",
+	"register_section_account": "Account",
+	"register_section_notifications": "Notifications",
+	"register_label_password_confirm": "Confirm password",
+	"register_pw_hint": "At least 8 characters.",
+	"register_pw_ok": "At least 8 characters. ✓",
+	"register_pw_match_ok": "Passwords match.",
+	"register_pw_match_no": "The two passwords don't match yet.",
+	"register_notify_title": "Notify me",
+	"register_notify_desc": "when someone mentions me in a comment or replies to one of my comments.",
+	"register_btn_loading": "Creating …",
+	"register_already_have_account": "Already have an account?",
+	"register_sign_in": "Sign in",
 	"login_registered_success": "Your account was created successfully. Sign in now.",
 	"admin_tab_invites": "Invites",
 	"admin_invites_list_title": "Invites",

frontend/messages/es.json

@@ -658,6 +658,22 @@
 	"register_invalid_code": "Enlace de invitación inválido",
 	"register_invalid_code_desc": "Este enlace de invitación no es válido, ya ha sido utilizado o ha expirado. Contacta al administrador.",
 	"register_success": "Tu cuenta ha sido creada. Ahora puedes iniciar sesión.",
+	"register_eyebrow": "Un proyecto familiar",
+	"register_hero_headline": "Qué bueno que estés aquí.",
+	"register_hero_subtext": "Juntos conservamos cartas y documentos – para la familia, ahora y en el futuro.",
+	"register_section_about": "Sobre ti",
+	"register_section_account": "Cuenta",
+	"register_section_notifications": "Notificaciones",
+	"register_label_password_confirm": "Confirmar contraseña",
+	"register_pw_hint": "Al menos 8 caracteres.",
+	"register_pw_ok": "Al menos 8 caracteres. ✓",
+	"register_pw_match_ok": "Las contraseñas coinciden.",
+	"register_pw_match_no": "Las dos contraseñas aún no coinciden.",
+	"register_notify_title": "Notifícame",
+	"register_notify_desc": "cuando alguien me mencione en un comentario o responda a uno de mis comentarios.",
+	"register_btn_loading": "Creando …",
+	"register_already_have_account": "¿Ya tienes una cuenta?",
+	"register_sign_in": "Iniciar sesión",
 	"login_registered_success": "Tu cuenta fue creada con éxito. Inicia sesión ahora.",
 	"admin_tab_invites": "Invitaciones",
 	"admin_invites_list_title": "Invitaciones",