feat: add user selection flow

frontend/src/styles.css

@@ -21,3 +21,53 @@ h1 {
   margin-top: 0;
 }
 
+button,
+input {
+  font: inherit;
+}
+
+button {
+  padding: 0.65rem 1rem;
+  border: 0;
+  border-radius: 0.4rem;
+  color: white;
+  background: #2563eb;
+  cursor: pointer;
+}
+
+button:disabled,
+input:disabled {
+  cursor: not-allowed;
+  opacity: 0.65;
+}
+
+.secondary {
+  margin-top: 1rem;
+  color: #1f2937;
+  background: #e5e7eb;
+}
+
+.user-list {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.75rem;
+}
+
+form {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: 0.75rem;
+}
+
+input {
+  box-sizing: border-box;
+  width: 100%;
+  padding: 0.6rem;
+  border: 1px solid #9ca3af;
+  border-radius: 0.4rem;
+}
+
+.error {
+  color: #b91c1c;
+}