fix(#71): disable notification preferences when user has no email address

Profile page now greys out the notification checkboxes and save button when
the user has no email set, with a hint to add one first.

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

frontend/messages/de.json

@@ -304,6 +304,7 @@
 	"notification_prefs_heading": "Benachrichtigungen",
 	"notification_pref_reply": "E-Mail, wenn jemand auf meinen Kommentar antwortet",
 	"notification_pref_mention": "E-Mail, wenn jemand mich in einem Kommentar erwähnt",
+	"notification_prefs_no_email": "Bitte trage zuerst eine E-Mail-Adresse ein, um Benachrichtigungen zu erhalten.",
 	"notification_unread": "ungelesen",
 	"mention_btn_label": "Person erwähnen",
 	"mention_popup_empty": "Keine Nutzer gefunden"

frontend/messages/en.json

@@ -304,6 +304,7 @@
 	"notification_prefs_heading": "Notifications",
 	"notification_pref_reply": "Email when someone replies to my comment",
 	"notification_pref_mention": "Email when someone mentions me in a comment",
+	"notification_prefs_no_email": "Please add an email address above to receive notifications.",
 	"notification_unread": "unread",
 	"mention_btn_label": "Mention person",
 	"mention_popup_empty": "No users found"

frontend/messages/es.json

@@ -304,6 +304,7 @@
 	"notification_prefs_heading": "Notificaciones",
 	"notification_pref_reply": "Correo cuando alguien responde a mi comentario",
 	"notification_pref_mention": "Correo cuando alguien me menciona en un comentario",
+	"notification_prefs_no_email": "Por favor, añade una dirección de correo electrónico para recibir notificaciones.",
 	"notification_unread": "no leído",
 	"mention_btn_label": "Mencionar persona",
 	"mention_popup_empty": "No se encontraron usuarios"