feat: add device traffic metrics with gateway telemetry reporting and admin UI display

Add rx_bytes and tx_bytes fields to Device type and API responses. Add formatDataSize helper for human-readable byte formatting with units from B to TB. Add Received and Sent columns to devices table in admin UI with formatted traffic totals. Add traffic metrics display to device action panel.

Add TelemetrySnapshot and PeerTelemetry types for gateway runtime stats. Add gateway telemetry endpoint at POST /gateway

backend/internal/httpserver/router.go

@@ -41,6 +41,7 @@ func NewRouter(jwtSecret string, handlers Handlers) http.Handler {
 		r.Post("/auth/logout", handlers.Auth.Logout)
 		r.Post("/gateway-agent/bootstrap", handlers.Gateway.Bootstrap)
 		r.Get("/gateway-agent/{id}/sync", handlers.Gateway.AgentSyncBundle)
+		r.Post("/gateway-agent/{id}/telemetry", handlers.Gateway.Telemetry)
 
 		r.Group(func(r chi.Router) {
 			r.Use(AuthMiddleware(jwtSecret))