nodes: store and show host disk free; warn when low (node-26#41)

Checkin now carries disk_free_bytes/disk_total_bytes. Stored on the node doc
(not overwritten by older nodes that don't send them). Node page shows a Disk
row; node cards warn amber under 15% free, red under 5% or 2 GB. A full disk
stopped node-002 recording for 17 days while it showed Online.

Frontend unverified locally (no Node, #207); CI build typechecks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/components/NodeCard.tsx

@@ -1,6 +1,7 @@
 import Link from "next/link";
 import { StatusBadge } from "@/components/StatusBadge";
 import type { NodeRecord, SystemRecord } from "@/lib/types";
+import { diskState } from "@/lib/nodeDisk";
 
 interface Props {
   node: NodeRecord;
@@ -45,6 +46,15 @@ export function NodeCard({ node, system, linkToDetail = true }: Props) {
             <span className="text-ink-2">{lastSeen}</span>
           </div>
         </div>
+        {(() => {
+          const disk = diskState(node);
+          if (!disk || disk.level === "ok") return null;
+          return (
+            <div className={`mt-3 text-xs font-mono border-t border-line pt-2 ${disk.level === "critical" ? "text-sev-major" : "text-sev-moderate"}`}>
+              ⚠ Disk {disk.level === "critical" ? "nearly full" : "low"}: {disk.label}
+            </div>
+          );
+        })()}
         {!node.configured && (
           <div className="mt-3 text-xs text-indigo-400 font-mono border-t border-line pt-2">
             ⚠ Needs configuration