Remove completed newProfileDropdown flag and promote enabled path

[Copilot]

newProfileDropdown is marked completed with outcome kept, so the temporary bifurcation is no longer needed. This PR removes the flag wiring and keeps the new profile dropdown behavior as the only code path.

• Frontend behavior cleanup

  • UserProfileContainer now always renders UserProfile for authenticated users.
  • Header no longer gates theme-toggle visibility on newProfileDropdown; the toggle is rendered unconditionally.
  • Removed obsolete LegacyUserProfile branching from the container.

• Flag contract cleanup

  • Removed newProfileDropdown from frontend UI flag typings (UiFlags).
  • Removed newProfileDropdown from backend experimental flag key/type and env parsing map.
  • Removed newProfileDropdown from dev server flag defaults.

• Targeted regression coverage

  • Added focused tests for UserProfileContainer to assert:
    • profile UI renders when a user exists
    • nothing is rendered when no user exists

const UserProfileContainer = () => {
  const { user } = useAuthUser();
  if (!user) return null;
  return <UserProfile profile={user} />;
};