feat: Add real-time queue depth indicator to viewer UI

Implements a visual badge that displays the count of active work items (queued + currently processing) in the worker service. The badge appears next to the claude-mem logo and updates in real-time via SSE.

Features:
- Shows count of pending messages + active SDK generators
- Only displays when queueDepth > 0
- Subtle pulse animation for visual feedback
- Theme-aware styling

Backend changes:
- Added getTotalActiveWork() method to SessionManager
- Updated worker-service to broadcast queueDepth via SSE
- Enhanced processing status API endpoint

Frontend changes:
- Updated Header component to display queue bubble
- Enhanced useSSE hook to track queueDepth state
- Added CSS styling with pulse animation

Closes #122
Closes #96
Closes #97

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/ui/viewer/hooks/useSSE.ts

@@ -10,6 +10,7 @@ export function useSSE() {
   const [projects, setProjects] = useState<string[]>([]);
   const [isConnected, setIsConnected] = useState(false);
   const [isProcessing, setIsProcessing] = useState(false);
+  const [queueDepth, setQueueDepth] = useState(0);
   const eventSourceRef = useRef<EventSource | null>(null);
   const reconnectTimeoutRef = useRef<NodeJS.Timeout>();
 
@@ -83,8 +84,9 @@ export function useSSE() {
 
             case 'processing_status':
               if (typeof data.isProcessing === 'boolean') {
-                console.log('[SSE] Processing status:', data.isProcessing);
+                console.log('[SSE] Processing status:', data.isProcessing, 'Queue depth:', data.queueDepth);
                 setIsProcessing(data.isProcessing);
+                setQueueDepth(data.queueDepth || 0);
               }
               break;
           }
@@ -107,5 +109,5 @@ export function useSSE() {
     };
   }, []);
 
-  return { observations, summaries, prompts, projects, isProcessing, isConnected };
+  return { observations, summaries, prompts, projects, isProcessing, queueDepth, isConnected };
 }