feat: add interface traffic counters as fallback telemetry when conntrack flows unavailable

Add interface_traffic collection in agent to aggregate VM/LXC network counters by vmid/nic with tap/fwln/fwpr/fwbr interface ranking, implement collect_interface_traffic to select best interface per VM NIC and format as flow-like records with rx/tx bytes/packets, add collect_flow_diagnostics to capture conntrack binary path and kernel bridge/netfilter settings for debugging, update workload_insights endpoint

frontend/src/pages/Workloads.tsx

@@ -63,7 +63,9 @@ export function Workloads() {
                     <div key={index} className="rounded-md border border-border p-3">
                       <div>{String(flow.source)} → {String(flow.destination)}</div>
                       <div className="text-xs text-slate-500">{String(flow.protocol)}:{String(flow.port)} · {String(flow.bytes)} bytes · {String(flow.decision)}</div>
+                      {flow.protocol === "interface-counter" ? <div className="mt-1 text-xs text-slate-500">Interface: {String(flow.interface ?? "unknown")} · rx {String(flow.rx_bytes ?? 0)} · tx {String(flow.tx_bytes ?? 0)}</div> : null}
                       {Array.isArray(flow.ip_addresses) ? <div className="mt-1 text-xs text-slate-500">IPs: {flow.ip_addresses.join(", ")}</div> : null}
+                      {flow.note ? <div className="mt-1 text-xs text-slate-500">{String(flow.note)}</div> : null}
                     </div>
                   )) : <div className="rounded-md border border-border p-3 text-xs text-slate-500">No real traffic telemetry has been collected yet. Install the node agent or enable a flow source to populate this section.</div>}
                 </div>