Update invitation acceptance UI for clarity

Updated the invitation acceptance interface components. The 'Join Team' button text has been changed to reflect the user's email. This provides the user more clarity about which account they are accepting the invitation for. Other minor adjustments were made to the layout and heading level for consistency and improved readability.

apps/web/app/join/page.tsx

@@ -94,8 +94,11 @@ async function JoinTeamAccountPage({ searchParams }: Context) {
     invitation.account.slug,
   );
 
+  const email = auth.data.email ?? '';
+
   return (
     <AcceptInvitationContainer
+      email={email}
       inviteToken={token}
       invitation={invitation}
       paths={{