feat(auth): add sign up flow with email domain validation and password confirm

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

src/lib/auth.ts

@@ -8,6 +8,26 @@ export interface AuthSession {
   user: { id: string; email: string };
 }
 
+export async function signUp(email: string, password: string): Promise<void> {
+  if (!email.toLowerCase().endsWith(ALLOWED_DOMAIN)) {
+    throw new Error(`Only ${ALLOWED_DOMAIN} email addresses are allowed.`);
+  }
+
+  const response = await fetch(`${SUPABASE_URL}/auth/v1/signup`, {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+      'apikey': SUPABASE_ANON_KEY,
+    },
+    body: JSON.stringify({ email, password }),
+  });
+
+  if (!response.ok) {
+    const err = await response.json().catch(() => ({}));
+    throw new Error(err.error_description || err.message || 'Registration failed.');
+  }
+}
+
 export async function signIn(email: string, password: string): Promise<AuthSession> {
   if (!email.toLowerCase().endsWith(ALLOWED_DOMAIN)) {
     throw new Error(`Only ${ALLOWED_DOMAIN} email addresses are allowed.`);