fix: codex S2 review — race conditions, try/finally, scroll, compose reset

- channel switch clears messages immediately, prevents stale fetch overwrite
- auto-scroll only on NEW messages (not every poll cycle)
- ComposeBox keyed by channelId — resets draft on switch
- try/finally on all mutations — failed sends don't disable compose
- loadChannels no longer re-fetches on every channel select

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/colony/src/components/ComposeBox.tsx

@@ -30,16 +30,19 @@ export function ComposeBox({
   async function handleSend() {
     if (!content.trim() || sending) return;
     setSending(true);
-    await postMessage(channelId, {
-      content: content.trim(),
-      type: msgType,
-      reply_to: replyTo ?? undefined,
-    });
-    setContent("");
-    setMsgType("text");
-    onClearReply();
-    onMessageSent();
-    setSending(false);
+    try {
+      await postMessage(channelId, {
+        content: content.trim(),
+        type: msgType,
+        reply_to: replyTo ?? undefined,
+      });
+      setContent("");
+      setMsgType("text");
+      onClearReply();
+      onMessageSent();
+    } finally {
+      setSending(false);
+    }
   }
 
   return (