Redesign SNS quick login buttons

Show Naver/Kakao/Google as circular icon buttons with a 'SNS 간편로그인' label, and toggle the group based on configured connections.

Co-authored-by: Cursor <cursoragent@cursor.com>

styles.css

@@ -180,6 +180,101 @@ html[data-theme="light"] .topbar {
   border-color: rgba(3, 199, 90, 0.28);
 }
 
+.sns-login {
+  display: inline-flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 10px;
+  border-radius: 14px;
+  border: 1px solid var(--border);
+  background: rgba(255, 255, 255, 0.03);
+}
+
+.sns-label {
+  font-size: 12px;
+  color: var(--muted2);
+  margin-right: 4px;
+  white-space: nowrap;
+}
+
+.sns-btn {
+  width: 42px;
+  height: 42px;
+  border-radius: 999px;
+  border: 1px solid rgba(0, 0, 0, 0.08);
+  display: grid;
+  place-items: center;
+  cursor: pointer;
+  padding: 0;
+  user-select: none;
+  transition: transform 120ms ease, filter 120ms ease;
+}
+
+.sns-btn:hover {
+  transform: translateY(-1px);
+  filter: brightness(1.02);
+}
+
+.sns-btn:active {
+  transform: translateY(0);
+}
+
+.sns-btn:focus-visible {
+  outline: none;
+  box-shadow: var(--focus);
+}
+
+.sns-naver {
+  background: #03c75a;
+  border-color: rgba(3, 199, 90, 0.35);
+}
+
+.sns-letter {
+  font-weight: 900;
+  font-size: 18px;
+  color: #fff;
+  letter-spacing: -0.02em;
+}
+
+.sns-kakao {
+  background: #fae100;
+  border-color: rgba(250, 225, 0, 0.5);
+}
+
+.sns-kakao-bubble {
+  width: 16px;
+  height: 13px;
+  background: rgba(0, 0, 0, 0.82);
+  border-radius: 6px;
+  position: relative;
+}
+
+.sns-kakao-bubble::after {
+  content: "";
+  position: absolute;
+  bottom: -4px;
+  left: 4px;
+  width: 0;
+  height: 0;
+  border: 4px solid transparent;
+  border-top-color: rgba(0, 0, 0, 0.82);
+}
+
+.sns-google {
+  background: #ffffff;
+  border-color: rgba(66, 133, 244, 0.35);
+}
+
+.sns-google-g {
+  font-weight: 900;
+  font-size: 18px;
+  background: conic-gradient(from 210deg, #ea4335 0 25%, #fbbc05 25% 50%, #34a853 50% 75%, #4285f4 75% 100%);
+  -webkit-background-clip: text;
+  background-clip: text;
+  color: transparent;
+  letter-spacing: -0.04em;
+}
+
 .user {
   display: inline-flex;
   align-items: center;