Fix two lint errors from the P2 header work

A masked lint exit code (piped through tail) let these ship in 65a0f1a:
- DevicePanel: reset subHeader via the render-phase prev-state pattern instead
  of a setState-in-effect.
- AppShell useSubHeader: assign the latest-callback ref in an effect, not
  during render.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/components/computer/DevicePanel.tsx

@@ -3,7 +3,6 @@
 import { ChevronLeft, X } from "lucide-react";
 import { useQueryStates } from "nuqs";
 import {
-  useEffect,
   useRef,
   useState,
   type CSSProperties,
@@ -39,14 +38,16 @@ export function DevicePanel({ agent }: { agent: Agent }) {
   // A drilled sub-screen registers its back-header here (see useSubHeader); the
   // single panel header morphs to back+title instead of app name + close.
   const [subHeader, setSubHeader] = useState<SubHeader | null>(null);
+  // Reset the registered sub-header when the open app changes (render-phase
+  // prev-state pattern — no effect).
+  const [seenApp, setSeenApp] = useState(app);
+  if (seenApp !== app) {
+    setSeenApp(app);
+    setSubHeader(null);
+  }
   const open = app !== null;
   const onHome = app === "home" || app === null;
 
-  // Switching apps (or going home) resets any registered sub-header.
-  useEffect(() => {
-    setSubHeader(null);
-  }, [app]);
-
   // Opening mounts content immediately; closing keeps it mounted through the
   // collapse transition (flex-basis when leaving phone/tablet, flex-grow when
   // leaving full), then unmounts.

frontend/src/components/computer/apps/AppShell.tsx

@@ -21,7 +21,10 @@ export function useSubHeader(
 ) {
   const setSub = useContext(SubHeaderContext);
   const backRef = useRef(onBack);
-  backRef.current = onBack;
+  // Keep the latest callback in the ref (in an effect, not during render).
+  useEffect(() => {
+    backRef.current = onBack;
+  });
   useEffect(() => {
     if (!active) return;
     setSub({ title, onBack: () => backRef.current() });