feat(server,frontend): real-time sync architecture with message accumulator

- Add message-accumulator.js: cascades diff-based message accumulation
- Add 3-second cascade polling with broadcastToAll (was undefined!)
- Add /api/bridge/approve endpoint: tries accept/reject Step→Command→Terminal
- Add persistent approve/reject buttons in chat header toolbar
- Frontend: loadSessionMessages (trajectory + accumulated), applyNewMessages (WS push)
- Status change detection: _prevStatusKey tracking prevents unnecessary re-renders
- actionInProgress flag prevents DOM replacement during button fetch
- Known issues: Trajectory 341 hard limit, Cascade no command-approval state

public/css/style.css

@@ -182,6 +182,37 @@ body {
   padding: 0 8px 8px;
 }
 
+/* 프로젝트 그룹 헤더 */
+.project-group-header {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px 12px 4px;
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.project-group-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  flex-shrink: 0;
+}
+
+/* 이전 메시지 로더 */
+.load-more-indicator {
+  text-align: center;
+  color: var(--text-muted);
+  font-size: 11px;
+  padding: 12px 0;
+  opacity: 0.6;
+  border-bottom: 1px solid var(--border-subtle);
+  margin-bottom: 8px;
+}
+
 /* Session Card */
 .session-card {
   display: flex;
@@ -392,6 +423,56 @@ body {
 .chat-actions {
   display: flex;
   gap: 6px;
+  align-items: center;
+}
+
+.approve-controls {
+  display: flex;
+  gap: 4px;
+  margin-left: 8px;
+  padding-left: 8px;
+  border-left: 1px solid var(--border);
+}
+
+.btn-approve,
+.btn-reject {
+  font-size: 12px;
+  padding: 4px 10px;
+  border: 1px solid transparent;
+  border-radius: 6px;
+  cursor: pointer;
+  font-weight: 500;
+  transition: all 0.2s;
+}
+
+.btn-approve {
+  background: rgba(34, 197, 94, 0.15);
+  color: #22c55e;
+  border-color: rgba(34, 197, 94, 0.3);
+}
+
+.btn-approve:hover {
+  background: rgba(34, 197, 94, 0.3);
+}
+
+.btn-approve:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
+.btn-reject {
+  background: rgba(239, 68, 68, 0.15);
+  color: #ef4444;
+  border-color: rgba(239, 68, 68, 0.3);
+}
+
+.btn-reject:hover {
+  background: rgba(239, 68, 68, 0.3);
+}
+
+.btn-reject:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
 }
 
 /* Chat Messages */