chore(dashboard): address copilot review nits on #16861

- App.tsx doc comment: replace stale ChatPageHost reference with
  'persistent chat host block rendered inline near the bottom of this
  file' so readers can find the actual code.
- App.tsx persistent host: show a small spinner on /chat while plugin
  manifests are loading instead of a blank content area.  Direct
  /chat deep-links used to paint empty for up to ~2s in the worst
  case (plugin-registration safety timeout) because both the route
  sink (null) and the persistent host (!pluginsLoading gate) render
  nothing during that window.  Non-chat routes stay empty as before.
- ChatPage.tsx: rename setter to match the 'raw' state — useState
  now destructures as [mobilePanelOpenRaw, setMobilePanelOpenRaw],
  and all four call sites (closeMobilePanel, matchMedia listener,
  open-button onClick, plus destructure) updated accordingly.  No
  behavior change; matches the 'raw vs derived' convention the
  original comment set up.

web/src/App.tsx

@@ -80,11 +80,12 @@ const CHAT_NAV_ITEM: NavItem = {
 
 /**
  * Built-in routes except /chat.  Chat is rendered persistently (outside
- * <Routes>) when embedded — see ChatPageHost below — so the PTY child,
- * WebSocket, and xterm instance survive when the user visits another tab
- * and comes back.  A `display:none` toggle hides the terminal without
- * unmounting.  Routing still owns the URL so /chat deep-links, browser
- * back/forward, and nav highlight keep working.
+ * <Routes>) when embedded — see the persistent chat host block rendered
+ * inline near the bottom of this file — so the PTY child, WebSocket,
+ * and xterm instance survive when the user visits another tab and comes
+ * back.  A `display:none` toggle hides the terminal without unmounting.
+ * Routing still owns the URL so /chat deep-links, browser back/forward,
+ * and nav highlight keep working.
  */
 const BUILTIN_ROUTES_CORE: Record<string, ComponentType> = {
   "/": RootRedirect,
@@ -578,17 +579,37 @@ export default function App() {
                   or idle-disconnect after N minutes hidden; neither is
                   shipped today.
                 */}
-                {embeddedChat && !pluginsLoading && !chatOverriddenByPlugin && (
-                  <div
-                    data-chat-active={isChatRoute ? "true" : "false"}
-                    className={cn(
-                      "min-h-0 min-w-0",
-                      isChatRoute ? "flex flex-1 flex-col" : "hidden",
-                    )}
-                    aria-hidden={!isChatRoute}
-                  >
-                    <ChatPage isActive={isChatRoute} />
-                  </div>
+                {embeddedChat && !chatOverriddenByPlugin && (
+                  pluginsLoading ? (
+                    // Direct /chat deep-link: plugin manifests haven't resolved
+                    // yet, so we can't tell if a plugin is going to claim this
+                    // route.  Show a lightweight placeholder instead of a
+                    // blank page.  Typical wait is <50ms; worst case is the
+                    // 2s plugin-registration safety timeout.
+                    isChatRoute ? (
+                      <div
+                        className="flex min-h-0 min-w-0 flex-1 items-center justify-center"
+                        aria-busy="true"
+                        aria-live="polite"
+                      >
+                        <div className="flex items-center gap-2 text-sm text-muted-foreground">
+                          <Loader2 className="h-4 w-4 animate-spin" aria-hidden />
+                          <span>Loading chat…</span>
+                        </div>
+                      </div>
+                    ) : null
+                  ) : (
+                    <div
+                      data-chat-active={isChatRoute ? "true" : "false"}
+                      className={cn(
+                        "min-h-0 min-w-0",
+                        isChatRoute ? "flex flex-1 flex-col" : "hidden",
+                      )}
+                      aria-hidden={!isChatRoute}
+                    >
+                      <ChatPage isActive={isChatRoute} />
+                    </div>
+                  )
                 )}
               </div>
               <PluginSlot name="post-main" />

web/src/pages/ChatPage.tsx

@@ -128,11 +128,11 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   // /chat re-runs the effect (derived flips back to true) and re-locks.
   // Keying on the raw state would leak the body.overflow="hidden" across
   // tabs because the dep wouldn't change on tab switch.
-  const [mobilePanelOpenRaw, setMobilePanelOpen] = useState(false);
+  const [mobilePanelOpenRaw, setMobilePanelOpenRaw] = useState(false);
   const mobilePanelOpen = isActive && mobilePanelOpenRaw;
   const { setEnd } = usePageHeader();
   const { t } = useI18n();
-  const closeMobilePanel = useCallback(() => setMobilePanelOpen(false), []);
+  const closeMobilePanel = useCallback(() => setMobilePanelOpenRaw(false), []);
   const modelToolsLabel = useMemo(
     () => `${t.app.modelToolsSheetTitle} ${t.app.modelToolsSheetSubtitle}`,
     [t.app.modelToolsSheetSubtitle, t.app.modelToolsSheetTitle],
@@ -174,7 +174,7 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   useEffect(() => {
     const mql = window.matchMedia("(min-width: 1024px)");
     const onChange = (e: MediaQueryListEvent) => {
-      if (e.matches) setMobilePanelOpen(false);
+      if (e.matches) setMobilePanelOpenRaw(false);
     };
     mql.addEventListener("change", onChange);
     return () => mql.removeEventListener("change", onChange);
@@ -194,7 +194,7 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
     setEnd(
       <button
         type="button"
-        onClick={() => setMobilePanelOpen(true)}
+        onClick={() => setMobilePanelOpenRaw(true)}
         className={cn(
           "inline-flex items-center gap-1.5 rounded border border-current/20",
           "px-2 py-1 text-[0.65rem] font-medium tracking-wide normal-case",