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/lib/nodeDisk.ts

@@ -0,0 +1,39 @@
+import type { NodeRecord } from "@/lib/types";
+
+// Free disk on the node's host (node-26#41). A full disk stopped node-002
+// recording for 17 days while every surface still showed it Online, so low
+// space is flagged wherever a node is shown.
+
+export type DiskLevel = "ok" | "low" | "critical";
+
+export interface DiskState {
+  level: DiskLevel;
+  freePct: number;
+  label: string; // e.g. "43.2 GB free of 98 GB (44%)"
+}
+
+const LOW_PCT = 15;
+const CRITICAL_PCT = 5;
+const CRITICAL_BYTES = 2 * 1024 ** 3; // a 2 GB floor regardless of disk size
+
+function gb(bytes: number): string {
+  const v = bytes / 1024 ** 3;
+  return `${v >= 10 ? v.toFixed(0) : v.toFixed(1)} GB`;
+}
+
+/** null when the node has never reported (older node software). */
+export function diskState(node: Pick<NodeRecord, "disk_free_bytes" | "disk_total_bytes">): DiskState | null {
+  const free = node.disk_free_bytes;
+  const total = node.disk_total_bytes;
+  if (free == null || !total) return null;
+  const freePct = Math.max(0, Math.min(100, (free / total) * 100));
+  const level: DiskLevel =
+    freePct < CRITICAL_PCT || free < CRITICAL_BYTES ? "critical" : freePct < LOW_PCT ? "low" : "ok";
+  return { level, freePct, label: `${gb(free)} free of ${gb(total)} (${Math.round(freePct)}%)` };
+}
+
+export const DISK_TEXT_CLASS: Record<DiskLevel, string> = {
+  ok: "text-ink-2",
+  low: "text-sev-moderate",
+  critical: "text-sev-major",
+};