Discord admin Add form: say so when the system already plays in that channel (#213)

Re-adding the same system to a voice channel returns the running stream
untouched (already_on). The form closed as if a stream had been added and
the new label/bot was silently dropped; it now stays open and explains.

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

drb-frontend/components/discord/StreamsPanel.tsx

@@ -71,7 +71,7 @@ function AddChannelForm({
     setSaving(true);
     setError(null);
     try {
-      await c2api.startDiscordStream({
+      const res = await c2api.startDiscordStream({
         guild_id: guildId,
         channel_id: channelId.trim(),
         system_id: systemId,
@@ -79,6 +79,16 @@ function AddChannelForm({
         channel_name: channelName.trim() || undefined,
         preferred_token_id: botId || undefined,
       });
+      if (res?.already_on) {
+        // Same system re-added to a channel that already plays it: c2-core
+        // returns that stream untouched (label, mode and bot kept). Say so
+        // instead of closing as if a stream had been added.
+        setError({
+          message: `${res.system_name || "That system"} is already playing in this voice channel, so nothing was added or changed. Pick another system to add a second stream, or Remove the existing row to change its label or bot.`,
+          invite: [],
+        });
+        return;
+      }
       onDone();
     } catch (err) {
       setError(parseC2Error(err));