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/types.ts

@@ -82,6 +82,9 @@ export interface NodeRecord {
   /** The dongle OP25 is actually using, per the node. */
   op25_sdr_serial?: string | null;
   sdr_count?: number | null; // null = never reported
+  /** Host disk the node records to (node-26#41); null/absent = not reported. */
+  disk_free_bytes?: number | null;
+  disk_total_bytes?: number | null;
   /** node-26#24: conventional channels from the last checkin; [] on a trunked system. */
   radio_channels?: RadioChannel[] | null;
   enforce_override_timeout?: boolean;