Picker review fixes: validate stream_id on stop; accurate offline + no-system copy

- POST /discord/nodes/{id}/streams/stop: stream_id must be guild:channel
  snowflakes, so a malformed id is a 422 instead of a Firestore path error.
- Offline footer said channels resume on reconnect; ad-hoc ones stop once
  the node is gone past the lease grace (reconcile_once). Say so.
- Picker: say why Start is disabled when the node has no system.

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

drb-frontend/components/NodeDiscordRadio.tsx

@@ -271,6 +271,11 @@ function DiscordPicker({
             </fieldset>
           )}
 
+          {!options.system_id && (
+            <p className="text-xs text-sev-moderate">
+              This node is not listening to a system yet. Assign it one before adding a radio channel.
+            </p>
+          )}
           {atNodeCap && !channelOnHere && (
             <p className="text-xs text-sev-moderate">
               This node already carries {options.limits.node_used} of {options.limits.node_max} radio channels. Stop one
@@ -439,7 +444,7 @@ export function NodeDiscordRadio({ nodeId }: { nodeId: string }) {
       {actionError && <p role="alert" className="px-4 py-2 text-xs text-sev-major border-t border-line">{actionError}</p>}
       {options && !options.node_online && streams.length > 0 && (
         <p className="px-4 py-2 text-xs text-ink-muted bg-raised border-t border-line">
-          This node is offline — its channels resume when it reconnects. If it stays down, persistent ones move to another node on the same system.
+          This node is offline — its channels resume if it reconnects shortly. If it stays down, ad-hoc ones stop and persistent ones move to another node on the same system.
         </p>
       )}