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/lib/useRegistrationOpen.ts

@@ -0,0 +1,45 @@
+import { useEffect, useState } from "react";
+import { REGISTRATION_OPENS_AT } from "./opening";
+
+interface RegistrationOpenState {
+	isOpen: boolean;
+	days: number;
+	hours: number;
+	minutes: number;
+	seconds: number;
+}
+
+function compute(): RegistrationOpenState {
+	const diff = REGISTRATION_OPENS_AT.getTime() - Date.now();
+	if (diff <= 0) {
+		return { isOpen: true, days: 0, hours: 0, minutes: 0, seconds: 0 };
+	}
+	const totalSeconds = Math.floor(diff / 1000);
+	const days = Math.floor(totalSeconds / 86400);
+	const hours = Math.floor((totalSeconds % 86400) / 3600);
+	const minutes = Math.floor((totalSeconds % 3600) / 60);
+	const seconds = totalSeconds % 60;
+	return { isOpen: false, days, hours, minutes, seconds };
+}
+
+/**
+ * Returns live countdown state to {@link REGISTRATION_OPENS_AT}.
+ * Ticks every second; clears the interval once registration is open.
+ */
+export function useRegistrationOpen(): RegistrationOpenState {
+	const [state, setState] = useState<RegistrationOpenState>(compute);
+
+	useEffect(() => {
+		if (state.isOpen) return;
+
+		const id = setInterval(() => {
+			const next = compute();
+			setState(next);
+			if (next.isOpen) clearInterval(id);
+		}, 1000);
+
+		return () => clearInterval(id);
+	}, [state.isOpen]);
+
+	return state;
+}