Add /study/signup page with auth form, benefits, and start-here style link

src/App.css

@@ -1891,6 +1891,85 @@
   padding: 0;
 }
 
+/* ── Study Signup Page ── */
+.study-signup-page-card {
+  max-width: 680px;
+}
+
+.study-signup-form {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  margin-top: 1rem;
+}
+
+.study-signup-form label {
+  font-family: var(--brand-font-body);
+  font-size: 0.82rem;
+  font-weight: 600;
+  color: var(--brand-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.07em;
+  margin-top: 0.25rem;
+}
+
+.study-signup-form input {
+  background: rgba(255, 255, 255, 0.04);
+  border: 1px solid rgba(201, 168, 76, 0.25);
+  border-radius: 8px;
+  padding: 0.7rem 0.9rem;
+  font-family: var(--brand-font-body);
+  font-size: 1rem;
+  color: var(--brand-warm-white);
+  outline: none;
+  transition: border-color 180ms;
+}
+
+.study-signup-form input:focus {
+  border-color: rgba(201, 168, 76, 0.6);
+}
+
+.study-signup-error {
+  font-size: 0.9rem !important;
+  color: #e57373 !important;
+  margin: 0.25rem 0 0 !important;
+}
+
+.study-signup-success {
+  font-size: 1.05rem !important;
+  color: #81c784 !important;
+  margin: 0.5rem 0 1rem !important;
+}
+
+.study-signup-actions {
+  display: flex;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+  margin-top: 0.75rem;
+}
+
+.study-signup-switch {
+  font-size: 0.88rem !important;
+  color: var(--brand-muted) !important;
+  margin: 0.5rem 0 0 !important;
+}
+
+.study-signup-toggle {
+  background: none;
+  border: none;
+  cursor: pointer;
+  padding: 0;
+  font-family: var(--brand-font-body);
+  font-size: 0.88rem;
+  color: rgba(201, 168, 76, 0.85);
+  text-decoration: underline;
+  text-underline-offset: 2px;
+}
+
+.study-signup-toggle:hover {
+  color: var(--brand-gold);
+}
+
 .subscribe-card {
   max-width: 560px;
 }