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/services/worker/SessionManager.ts

@@ -256,6 +256,23 @@ export class SessionManager {
     return total;
   }
 
+  /**
+   * Get total active work (queued + currently processing)
+   * Counts both pending messages and items actively being processed by SDK agents
+   */
+  getTotalActiveWork(): number {
+    let total = 0;
+    for (const session of this.sessions.values()) {
+      // Count queued messages
+      total += session.pendingMessages.length;
+      // Count currently processing item (1 per active generator)
+      if (session.generatorPromise !== null) {
+        total += 1;
+      }
+    }
+    return total;
+  }
+
   /**
    * Check if any session is actively processing (has pending messages OR active generator)
    * Used for activity indicator to prevent spinner from stopping while SDK is processing