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/package.json

@@ -30,6 +30,7 @@
 		"@tanstack/react-start": "^1.141.1",
 		"@tanstack/router-plugin": "^1.141.1",
 		"better-auth": "catalog:",
+		"canvas-confetti": "^1.9.4",
 		"class-variance-authority": "^0.7.1",
 		"clsx": "^2.1.1",
 		"dotenv": "catalog:",
@@ -56,6 +57,7 @@
 		"@tanstack/react-router-devtools": "^1.141.1",
 		"@testing-library/dom": "^10.4.0",
 		"@testing-library/react": "^16.2.0",
+		"@types/canvas-confetti": "^1.9.0",
 		"@types/qrcode": "^1.5.6",
 		"@types/react": "19.2.7",
 		"@types/react-dom": "19.2.3",