Secondary SDR priority: C2 field, node command, dashboard section

Pairs with node-26 feat/secondary-sdr-priority. NodeRecord gains
secondary_sdr_priority (ordered; SDRs beyond OP25's run it top-down) and
secondary_sdr_running, both mirrored from the node's checkin.
PATCH /nodes/{id} accepts the priority, validates it, and sends it as a
'set_secondary_priority' MQTT command. A priority-only change never
re-pushes system config, because that restarts OP25. The node detail page
gets a 'Secondary SDRs' section (admin-editable) with enable, reorder,
save, and live Running / Waiting-for-SDR state from the checkin.

Verified: c2-core pytest 482 passed; frontend tsc --noEmit clean.

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

drb-frontend/lib/types.ts

@@ -53,7 +53,11 @@ export interface NodeRecord {
   hardware_preset?: string;
   ppm_override?: number | null;
   node_type?: string;
-  secondary_sdr_mode?: string;
+  secondary_sdr_mode?: string; // legacy; priority[0] on current nodes
+  /** Ordered decoders for the SDRs beyond OP25's, run top-down (node-26#9). */
+  secondary_sdr_priority?: string[];
+  /** What the node's last checkin reported actually running. */
+  secondary_sdr_running?: string[] | null;
   sdr_count?: number;
   enforce_override_timeout?: boolean;
   is_overridden?: boolean;