feat: add CLI Viewer Page with multi-pane layout and state management

- Implemented the CliViewerPage component for displaying CLI outputs in a configurable multi-pane layout.
- Integrated Zustand for state management, allowing for dynamic layout changes and tab management.
- Added layout options: single, split horizontal, split vertical, and 2x2 grid.
- Created viewerStore for managing layout, panes, and tabs, including actions for adding/removing panes and tabs.
- Added CoordinatorPage barrel export for easier imports.

ccw/frontend/src/components/layout/Sidebar.tsx

@@ -22,7 +22,6 @@ import {
   Clock,
   Zap,
   GitFork,
-  Activity,
   Shield,
   History,
   Server,
@@ -81,7 +80,6 @@ const navGroupDefinitions: NavGroupDef[] = [
       { path: '/lite-tasks', labelKey: 'navigation.main.liteTasks', icon: Zap },
       { path: '/orchestrator', labelKey: 'navigation.main.orchestrator', icon: Workflow },
       { path: '/coordinator', labelKey: 'navigation.main.coordinator', icon: GitFork },
-      { path: '/executions', labelKey: 'navigation.main.executions', icon: Activity },
       { path: '/loops', labelKey: 'navigation.main.loops', icon: RefreshCw },
       { path: '/history', labelKey: 'navigation.main.history', icon: Clock },
     ],