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/App.tsx

@@ -17,7 +17,7 @@ export function App() {
   const [paginatedSummaries, setPaginatedSummaries] = useState<Summary[]>([]);
   const [paginatedPrompts, setPaginatedPrompts] = useState<UserPrompt[]>([]);
 
-  const { observations, summaries, prompts, projects, isProcessing, isConnected } = useSSE();
+  const { observations, summaries, prompts, projects, isProcessing, queueDepth, isConnected } = useSSE();
   const { settings, saveSettings, isSaving, saveStatus } = useSettings();
   const { stats, refreshStats } = useStats();
   const { preference, resolvedTheme, setThemePreference } = useTheme();
@@ -96,6 +96,7 @@ export function App() {
           onSettingsToggle={toggleSidebar}
           sidebarOpen={sidebarOpen}
           isProcessing={isProcessing}
+          queueDepth={queueDepth}
           themePreference={preference}
           onThemeChange={setThemePreference}
         />