batch3 T7: pane components — PaneShell, ChatPane, FileBrowserPane, PaneTab, Workspace

- PaneShell: per-pane chrome (kind label + close)
- ChatPane: extracts message+input rendering, subscribes to useSessionStream
- FileBrowserPane: tree + filter (debounced 100ms) + inline viewer via Shiki
- PaneTab: tab with kind icon + context menu (Split, Close, Close others,
  Close to right, Close all) via shadcn ContextMenu
- Workspace: tab strip + pane grid (CSS grid repeat(N,1fr)), native HTML5
  drag-to-reorder, "+" button (disabled at 5), subscribes to
  open_file_in_browser (focus existing file-browser pane or spawn one)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,39 @@
+import { useEffect, useRef } from 'react';
+import { toast } from 'sonner';
+import { api } from '@/api/client';
+import { useSessionStream } from '@/hooks/useSessionStream';
+import { MessageList } from '@/components/MessageList';
+import { ChatInput } from '@/components/ChatInput';
+
+interface Props {
+  sessionId: string;
+}
+
+export function ChatPane({ sessionId }: Props) {
+  const stream = useSessionStream(sessionId);
+  const lastErrorRef = useRef<string | null>(null);
+
+  // Surface stream errors via toast — matches Session.tsx behavior.
+  useEffect(() => {
+    if (stream.error && stream.error !== lastErrorRef.current) {
+      lastErrorRef.current = stream.error;
+      toast.error(stream.error);
+    }
+    if (!stream.error) {
+      lastErrorRef.current = null;
+    }
+  }, [stream.error]);
+
+  async function handleSend(content: string) {
+    await api.messages.send(sessionId, content);
+  }
+
+  const streaming = stream.messages.some((m) => m.status === 'streaming');
+
+  return (
+    <div className="flex flex-col h-full min-h-0">
+      <MessageList messages={stream.messages} sessionId={sessionId} />
+      <ChatInput disabled={streaming} onSend={handleSend} />
+    </div>
+  );
+}