feat(register): show invite-only error when no code param is present

Visiting /register without a code now shows a friendly error card
explaining the archive is invite-only, instead of the empty form.

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

frontend/messages/en.json

@@ -657,6 +657,8 @@
 	"register_btn_submit": "Create account",
 	"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_invite_only": "The archive is invite-only.",
+	"register_invite_only_desc": "You need a personal invite link to create an account. Please get in touch with a family member.",
 	"register_success": "Your account has been created. You can now sign in.",
 	"register_eyebrow": "A family project",
 	"register_hero_headline": "Glad you're here.",