feat(21-04): create ChatPanel shell and wire into Layout and main.tsx

- ChatPanel: 380px right-side drawer with transition-[width] and hidden md:flex
- Two-column skeleton: 160px conversation list + flex thread area with ChatInput
- Layout: import ChatPanel, MessageSquare, useChatPanel; add chat toggle button
- Layout: useEffect closes PropertiesPanel when chatOpen becomes true
- Layout: ChatPanel rendered before PropertiesPanel in flex row
- main.tsx: ChatPanelProvider wrapping app inside PanelProvider

ui/src/components/ChatPanel.tsx

@@ -0,0 +1,63 @@
+import { X } from "lucide-react";
+import { useChatPanel } from "../context/ChatPanelContext";
+import { ChatInput } from "./ChatInput";
+import { Button } from "@/components/ui/button";
+import { ScrollArea } from "@/components/ui/scroll-area";
+
+export function ChatPanel() {
+  const { chatOpen, setChatOpen } = useChatPanel();
+
+  return (
+    <aside
+      aria-label="Chat"
+      className="hidden md:flex overflow-hidden transition-[width] duration-100 ease-out flex-shrink-0 border-l border-border flex-col bg-background"
+      style={{ width: chatOpen ? 380 : 0 }}
+    >
+      {/* Header */}
+      <div className="flex items-center justify-between border-b border-border px-4 py-2 min-w-[380px]">
+        <span className="text-sm font-medium">Chat</span>
+        <Button
+          variant="ghost"
+          size="icon"
+          className="h-6 w-6"
+          onClick={() => setChatOpen(false)}
+          aria-label="Close chat"
+        >
+          <X className="h-4 w-4" />
+        </Button>
+      </div>
+
+      {/* Two-column layout: conversation list (left) + thread (right) */}
+      <div className="flex flex-1 min-h-0 min-w-[380px]">
+        {/* Left column: conversation list -- placeholder for Plan 05 */}
+        <div className="w-[160px] flex-shrink-0 border-r border-border bg-card overflow-hidden">
+          <div className="p-3 text-center text-xs text-muted-foreground">
+            No conversations yet
+          </div>
+        </div>
+
+        {/* Right column: message thread + input */}
+        <div className="flex flex-1 flex-col min-w-0">
+          <ScrollArea className="flex-1 p-3">
+            {/* Messages placeholder -- wired in Plan 05 */}
+            <div className="flex items-center justify-center h-full">
+              <p className="text-sm text-muted-foreground">
+                Send a message to start this conversation.
+              </p>
+            </div>
+          </ScrollArea>
+
+          {/* Input area */}
+          <div className="border-t border-border px-3 py-2">
+            <ChatInput
+              onSend={(content) => {
+                // TODO: Wire to API in Plan 05
+                console.log("send:", content);
+              }}
+            />
+          </div>
+        </div>
+      </div>
+    </aside>
+  );
+}

ui/src/components/Layout.tsx

@@ -1,11 +1,12 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { useQuery } from "@tanstack/react-query";
-import { BookOpen, Moon, Settings, Sun } from "lucide-react";
+import { BookOpen, MessageSquare, Moon, Settings, Sun } from "lucide-react";
 import { Link, Outlet, useLocation, useNavigate, useParams } from "@/lib/router";
 import { CompanyRail } from "./CompanyRail";
 import { Sidebar } from "./Sidebar";
 import { InstanceSidebar } from "./InstanceSidebar";
 import { BreadcrumbBar } from "./BreadcrumbBar";
+import { ChatPanel } from "./ChatPanel";
 import { PropertiesPanel } from "./PropertiesPanel";
 import { CommandPalette } from "./CommandPalette";
 import { NewIssueDialog } from "./NewIssueDialog";
@@ -19,6 +20,7 @@ import { DevRestartBanner } from "./DevRestartBanner";
 import { useDialog } from "../context/DialogContext";
 import { GeneralSettingsProvider } from "../context/GeneralSettingsContext";
 import { usePanel } from "../context/PanelContext";
+import { useChatPanel } from "../context/ChatPanelContext";
 import { useCompany } from "../context/CompanyContext";
 import { useSidebar } from "../context/SidebarContext";
 import { useTheme, THEME_META } from "../context/ThemeContext";
@@ -51,7 +53,8 @@ function readRememberedInstanceSettingsPath(): string {
 export function Layout() {
   const { sidebarOpen, setSidebarOpen, toggleSidebar, isMobile } = useSidebar();
   const { openNewIssue, openOnboarding } = useDialog();
-  const { togglePanelVisible } = usePanel();
+  const { togglePanelVisible, setPanelVisible } = usePanel();
+  const { chatOpen, toggleChat } = useChatPanel();
   const {
     companies,
     loading: companiesLoading,
@@ -150,6 +153,13 @@ export function Layout() {
 
   const togglePanel = togglePanelVisible;
 
+  // Close PropertiesPanel when chat panel opens
+  useEffect(() => {
+    if (chatOpen) {
+      setPanelVisible(false);
+    }
+  }, [chatOpen, setPanelVisible]);
+
   useCompanyPageMemory();
 
   useKeyboardShortcuts({
@@ -397,6 +407,21 @@ export function Layout() {
                     <Settings className="h-4 w-4" />
                   </Link>
                 </Button>
+                <Tooltip>
+                  <TooltipTrigger asChild>
+                    <Button
+                      type="button"
+                      variant="ghost"
+                      size="icon-sm"
+                      className="hidden md:inline-flex text-muted-foreground shrink-0"
+                      onClick={toggleChat}
+                      aria-label={chatOpen ? "Close chat" : "Open chat"}
+                    >
+                      <MessageSquare className="h-4 w-4" />
+                    </Button>
+                  </TooltipTrigger>
+                  <TooltipContent>{chatOpen ? "Close chat" : "Open chat"}</TooltipContent>
+                </Tooltip>
                 <Button
                   type="button"
                   variant="ghost"
@@ -439,6 +464,7 @@ export function Layout() {
                 <Outlet />
               )}
             </main>
+            <ChatPanel />
             <PropertiesPanel />
           </div>
         </div>

ui/src/main.tsx

@@ -9,6 +9,7 @@ import { CompanyProvider } from "./context/CompanyContext";
 import { LiveUpdatesProvider } from "./context/LiveUpdatesProvider";
 import { BreadcrumbProvider } from "./context/BreadcrumbContext";
 import { PanelProvider } from "./context/PanelContext";
+import { ChatPanelProvider } from "./context/ChatPanelContext";
 import { SidebarProvider } from "./context/SidebarContext";
 import { DialogProvider } from "./context/DialogContext";
 import { ToastProvider } from "./context/ToastContext";
@@ -48,11 +49,13 @@ createRoot(document.getElementById("root")!).render(
                   <BreadcrumbProvider>
                     <SidebarProvider>
                       <PanelProvider>
-                        <PluginLauncherProvider>
-                          <DialogProvider>
-                            <App />
-                          </DialogProvider>
-                        </PluginLauncherProvider>
+                        <ChatPanelProvider>
+                          <PluginLauncherProvider>
+                            <DialogProvider>
+                              <App />
+                            </DialogProvider>
+                          </PluginLauncherProvider>
+                        </ChatPanelProvider>
                       </PanelProvider>
                     </SidebarProvider>
                   </BreadcrumbProvider>