feat: Migrate from Socket.IO to Server-Sent Events (SSE)

- Replace Socket.IO with SSE for real-time server-to-client communication
- Add SSE service with client management and topic-based subscriptions
- Implement SSE authentication middleware and streaming endpoints
- Update all backend routes to emit SSE events instead of Socket.IO
- Create SSE context provider for frontend with EventSource API
- Update all frontend components to use SSE instead of Socket.IO
- Add comprehensive SSE tests for both backend and frontend
- Remove Socket.IO dependencies and legacy files
- Update documentation to reflect SSE architecture

Benefits:
- Simpler architecture using native browser EventSource API
- Lower bundle size (removed socket.io-client dependency)
- Better compatibility with reverse proxies and load balancers
- Reduced resource usage for Raspberry Pi deployment
- Standard HTTP-based real-time communication

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/routes/tasks.js

@@ -71,6 +71,15 @@ router.post(
       description,
     });
 
+    // Emit SSE event for new task
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic("tasks", "taskUpdate", {
+        type: "new_task",
+        task,
+      });
+    }
+
     res.json(task);
   }),
 );
@@ -120,6 +129,15 @@ router.put(
         }
       );
 
+      // Emit SSE event for task completion
+      const sse = req.app.get("sse");
+      if (sse) {
+        sse.broadcastToTopic("tasks", "taskUpdate", {
+          type: "task_completed",
+          task,
+        });
+      }
+
       res.json({
         task,
         pointsAwarded: task.pointsAwarded || 10,