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/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",