feat(web/m7-user-mgmt): /register page + login link

Public /register form: username, optional display name, password +
confirm, optional invite token. Submits to POST /api/auth/register
via the new register() helper in auth/store.svelte.ts. On success
the server sets the session cookie and the frontend redirects to /;
on error the page shows a code-specific message (invite_required,
invite_invalid, username_taken, etc.).

Login page gets a "Don't have an account? Register" link below the
form for symmetry.

Tests cover form rendering, password-mismatch client-side rejection,
the happy-path submit + redirect, and the invite-invalid error
mapping.

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

web/src/routes/login/login.test.ts

@@ -46,6 +46,12 @@ describe('login page', () => {
     expect(screen.getByRole('button', { name: /sign in/i })).toBeInTheDocument();
   });
 
+  test('renders a link to the register page', () => {
+    render(LoginPage);
+    const link = screen.getByRole('link', { name: /register/i });
+    expect(link).toHaveAttribute('href', '/register');
+  });
+
   test('invalid credentials show inline error and clear password', async () => {
     (login as ReturnType<typeof vi.fn>).mockRejectedValue({
       code: 'invalid_credentials', message: 'bad creds', status: 401