feat(web-gateway,web-dashboard): WS ping/pong keepalive, vite build config

- Respond to WebSocket Ping frames with Pong for keepalive detection
- Refactor WS handler to share sender between broadcast and recv tasks
  via Arc<Mutex> (required for pong responses from recv task)
- Add vite production build config: source maps, manual chunk splitting
  for vendor/charts/query bundles

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web-dashboard/vite.config.ts

@@ -12,4 +12,16 @@ export default defineConfig({
       },
     },
   },
+  build: {
+    sourcemap: true,
+    rollupOptions: {
+      output: {
+        manualChunks: {
+          vendor: ['react', 'react-dom', 'react-router-dom'],
+          charts: ['lightweight-charts', 'recharts'],
+          query: ['@tanstack/react-query'],
+        },
+      },
+    },
+  },
 })