Secondary SDR panel: never present an unreported SDR count or run state as fact

QA (drb-qa-review) blockers: sdr_count defaulted to 1 for nodes that
never sent it, so the panel claimed 'reports 1 SDR (0 spare)'. It is now
None until reported, and the count is only quoted alongside a real
secondary_sdr_running report. Rows read 'Not reported' instead of
'Waiting for SDR' when the node hasn't said what's running (closes #187).

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

drb-frontend/lib/types.ts

@@ -58,7 +58,7 @@ export interface NodeRecord {
   secondary_sdr_priority?: string[];
   /** What the node's last checkin reported actually running. */
   secondary_sdr_running?: string[] | null;
-  sdr_count?: number;
+  sdr_count?: number | null; // null = never reported
   enforce_override_timeout?: boolean;
   is_overridden?: boolean;
   override_system_id?: string | null;