feat(landing): add login/register toggle and fix sign-in routing

Add toggle link between login and register forms in CTAModal allowing
users to switch authentication modes without closing the modal.
Fix UserMenu sign-in button to navigate to login instead of register.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

features/landing/frontend/src/components/CTAModal/CTAModal.css

@@ -397,6 +397,42 @@
   letter-spacing: 0.05em;
 }
 
+/* ============================================
+   AUTH TOGGLE (Login/Register switch)
+   ============================================ */
+
+.cta-auth-toggle {
+  margin-top: 1.25rem;
+  text-align: center;
+  font-size: 0.9rem;
+  color: rgba(255, 255, 255, 0.6);
+}
+
+.cta-auth-toggle-link {
+  background: none;
+  border: none;
+  padding: 0;
+  font: inherit;
+  font-weight: 600;
+  color: var(--modal-primary, #4ecdc4);
+  cursor: pointer;
+  text-decoration: underline;
+  text-decoration-style: dotted;
+  text-underline-offset: 2px;
+  transition: all 0.2s ease;
+}
+
+.cta-auth-toggle-link:hover {
+  color: var(--modal-gradient-to, #556ee6);
+  text-decoration-style: solid;
+}
+
+.cta-auth-toggle-link:focus-visible {
+  outline: 2px solid var(--modal-primary, #4ecdc4);
+  outline-offset: 2px;
+  border-radius: 2px;
+}
+
 /* ============================================
    SUCCESS STATE
    ============================================ */

features/landing/frontend/src/components/CTAModal/CTAModal.tsx

@@ -535,6 +535,37 @@ export default function CTAModal({ context, onClose }: CTAModalProps) {
                 >
                   {submissionState === 'submitting' ? config.submittingLabel : config.submitLabel}
                 </button>
+
+                {/* Toggle between login and register */}
+                {(context.type === 'login' || context.type === 'register') && (
+                  <p className="cta-auth-toggle">
+                    {context.type === 'login' ? (
+                      <>
+                        Don't have an account?{' '}
+                        <button
+                          type="button"
+                          className="cta-auth-toggle-link"
+                          onClick={() => openRegister(context.userType)}
+                          onMouseEnter={() => playSound('button-hover')}
+                        >
+                          Register
+                        </button>
+                      </>
+                    ) : (
+                      <>
+                        Already have an account?{' '}
+                        <button
+                          type="button"
+                          className="cta-auth-toggle-link"
+                          onClick={() => openLogin(context.userType)}
+                          onMouseEnter={() => playSound('button-hover')}
+                        >
+                          Sign In
+                        </button>
+                      </>
+                    )}
+                  </p>
+                )}
               </form>
             </>
           )}

features/landing/frontend/src/components/UserMenu.tsx

@@ -69,7 +69,7 @@ export default function UserMenu() {
 
   const handleSignIn = () => {
     playSound('button-click')
-    navigate(Routes.register())
+    navigate(Routes.login())
   }
 
   const handleSignOut = () => {