feat: replace header emoji buttons with reference Lucide-style icons

Add users, plus, and dual-monitor SVG icons with matching 32px circular button styles from Design Planning Hub.

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

frontend/src/index.css

@@ -115,6 +115,78 @@ body,
   gap: 12px;
 }
 
+/* F12 — 헤더 원형 아이콘 버튼 (team / + / 듀얼모니터) */
+.header-action-btn-new,
+.header-view-btn-new,
+.team-status-btn-new {
+  display: flex;
+  flex-shrink: 0;
+  align-items: center;
+  justify-content: center;
+  width: 32px;
+  height: 32px;
+  border: 1.5px solid #1a4d42;
+  border-radius: 50%;
+  color: #cef1eb;
+  cursor: pointer;
+  background: linear-gradient(180deg, #0d3f34 0%, #051f19 100%);
+  box-shadow: 0 0 0 1px #000, 0 2px 2px rgba(0, 0, 0, 0.6);
+  transition: all 0.2s ease-in-out;
+}
+
+.header-action-btn-new svg,
+.header-view-btn-new svg,
+.team-status-btn-new svg {
+  color: inherit;
+  filter: none;
+}
+
+.header-action-btn-new:hover,
+.header-view-btn-new:hover,
+.team-status-btn-new:hover {
+  color: #fff;
+  border-color: #36816d;
+  background: linear-gradient(180deg, #115244 0%, #072b23 100%);
+  transform: translateY(-1px);
+  box-shadow:
+    0 0 0 1px #000,
+    0 0 8px rgba(36, 204, 158, 0.4),
+    0 4px 8px rgba(0, 0, 0, 0.7);
+}
+
+.header-action-btn-new:active,
+.header-action-btn-new.active,
+.header-view-btn-new.active,
+.team-status-btn-new.active {
+  color: #cef1eb;
+  border-color: #1a4d42;
+  background: linear-gradient(180deg, #072b23 0%, #051f19 100%);
+  transform: translateY(0);
+  box-shadow: 0 0 0 1px #000, 0 1px 2px rgba(0, 0, 0, 0.6);
+}
+
+.dual-monitor-icon-wrap {
+  position: relative;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+.dual-monitor-icon-wrap .m-icon {
+  position: absolute;
+}
+
+.dual-monitor-icon-wrap .m-back {
+  top: 0;
+  left: -3px;
+  opacity: 0.5;
+}
+
+.dual-monitor-icon-wrap .m-front {
+  right: -3px;
+  bottom: 0;
+}
+
 /* F12 Design Planning Hub — 타이틀 */
 .main_tit {
   color: #bad8ca;