feat: gate registration behind 16 March 2026 19:00 opening date

- Add REGISTRATION_OPENS_AT const in lib/opening.ts as single source of truth
- Add useRegistrationOpen hook with live 1s countdown ticker
- Add CountdownBanner component (DD/HH/MM/SS) with canvas-confetti on open
- EventRegistrationForm shows countdown instead of form while closed
- Login page hides/disables signup while closed; login always available
- WatcherForm AccountModal guards signUp call as defence-in-depth

Closes #2

apps/web/src/components/registration/WatcherForm.tsx

@@ -12,6 +12,7 @@ import {
 	validatePhone,
 	validateTextField,
 } from "@/lib/registration";
+import { useRegistrationOpen } from "@/lib/useRegistrationOpen";
 import { client, orpc } from "@/utils/orpc";
 import { GiftSelector } from "./GiftSelector";
 import { GuestList } from "./GuestList";
@@ -49,6 +50,7 @@ function AccountModal({
 	const [password, setPassword] = useState("");
 	const [confirmPassword, setConfirmPassword] = useState("");
 	const [passwordError, setPasswordError] = useState<string | undefined>();
+	const { isOpen } = useRegistrationOpen();
 
 	const signupMutation = useMutation({
 		mutationFn: async () => {
@@ -83,6 +85,10 @@ function AccountModal({
 
 	const handleSignup = (e: React.FormEvent) => {
 		e.preventDefault();
+		if (!isOpen) {
+			toast.error("Registratie is nog niet open");
+			return;
+		}
 		if (password.length < 8) {
 			setPasswordError("Wachtwoord moet minstens 8 tekens zijn");
 			return;