feat(web): morphing send/stop/queue composer button

The composer's primary button now reflects generation state: Send when idle,
Stop while generating with an empty draft, and Queue while generating with a
draft typed (submitting queues it via the existing queue path). Stop is
click-only so a stray Enter never interrupts a run. ChatInput gains generating
+ onStop props.

BooChat: removes the separate centered "Stop generating" pill and wires
generating={streaming} + onStop={handleStop}. BooCoder: generating now keys on
sending || activeTaskId (the dispatch POST is too brief on its own), which also
fixes the queue gates that previously fired mid-run; onStop cancels the active
task via the new api.coder.cancelTask, and the input is no longer disabled while
a task runs so follow-ups can be queued.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/components/panes/CoderPane.tsx

@@ -581,6 +581,10 @@ export function CoderPane({
   const [queue, setQueue] = useState<string[]>([]);
   const queueProcessing = useRef(false);
   const inputRef = useRef<HTMLTextAreaElement>(null);
+  // The agent is "generating" during the dispatch POST (sending) AND while its
+  // task runs (activeTaskId). sending alone is too brief — it clears the moment
+  // dispatch returns — so queueing/stop must key on this combined signal.
+  const generating = sending || activeTaskId !== null;
 
   // Refresh pending changes when a message_complete arrives
   useEffect(() => {
@@ -760,24 +764,35 @@ export function CoderPane({
     }
   }, [sessionId, paneId, chatId, agentConfig, setMessages]);
 
-  // Drain queue when not busy
+  // Drain queue once the agent is idle (not just past the dispatch POST).
   useEffect(() => {
-    if (sending || queue.length === 0 || queueProcessing.current) return;
+    if (generating || queue.length === 0 || queueProcessing.current) return;
     queueProcessing.current = true;
     const next = queue[0]!;
     setQueue((prev) => prev.slice(1));
     sendOneMessage(next).finally(() => { queueProcessing.current = false; });
-  }, [sending, queue, sendOneMessage]);
+  }, [generating, queue, sendOneMessage]);
 
   const handleChatInputSend = useCallback(async (content: string) => {
     const text = content.trim();
     if (!text || !chatId) return;
-    if (sending) {
+    if (generating) {
       setQueue((prev) => [...prev, text]);
       return;
     }
     await sendOneMessage(text);
-  }, [sending, chatId, sendOneMessage]);
+  }, [generating, chatId, sendOneMessage]);
+
+  const handleStop = useCallback(async () => {
+    const taskId = activeTaskId;
+    if (!taskId) return;
+    try {
+      await api.coder.cancelTask(taskId);
+      setActiveTaskId(null); // optimistic; WS/poll terminal-state also clears it
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'stop failed');
+    }
+  }, [activeTaskId]);
 
   const handleChatInputSlash = useCallback(async (skillName: string, userMessage: string) => {
     if (!chatId) return;
@@ -867,9 +882,11 @@ export function CoderPane({
       {/* Composer + input */}
       <div className="shrink-0 border-t border-border">
         <ChatInput
-          disabled={sending || !chatId || chatPending}
+          disabled={!chatId || chatPending}
           projectId={projectPath ?? ''}
           onSend={handleChatInputSend}
+          generating={generating}
+          onStop={handleStop}
           onSlashCommand={handleChatInputSlash}
           slashGroups={slashGroups}
           chatId={chatId ?? undefined}