fix(terminal): only send resize from the focused client

Multiple clients viewing the same terminal would fight over PTY size
because every client sent resizes independently. Gate resize sends on
three focus levels: pane focus (split panes), screen focus (navigation),
and app visibility (browser window / mobile foreground). When a client
regains focus, clear the last reported size and trigger a reflow to
reclaim the PTY dimensions.

packages/app/src/hooks/use-app-visible.ts

@@ -0,0 +1,49 @@
+import { useEffect, useSyncExternalStore } from "react";
+import { AppState, Platform } from "react-native";
+import { getIsAppActivelyVisible } from "@/utils/app-visibility";
+
+let current = getIsAppActivelyVisible();
+const listeners = new Set<() => void>();
+
+function notify(): void {
+  const next = getIsAppActivelyVisible();
+  if (next === current) {
+    return;
+  }
+  current = next;
+  for (const listener of listeners) {
+    listener();
+  }
+}
+
+function subscribe(listener: () => void): () => void {
+  listeners.add(listener);
+  return () => listeners.delete(listener);
+}
+
+function getSnapshot(): boolean {
+  return current;
+}
+
+export function useAppVisible(): boolean {
+  useEffect(() => {
+    const appStateSubscription = AppState.addEventListener("change", notify);
+
+    if (Platform.OS === "web" && typeof document !== "undefined") {
+      document.addEventListener("visibilitychange", notify);
+      window.addEventListener("focus", notify);
+      window.addEventListener("blur", notify);
+    }
+
+    return () => {
+      appStateSubscription.remove();
+      if (Platform.OS === "web" && typeof document !== "undefined") {
+        document.removeEventListener("visibilitychange", notify);
+        window.removeEventListener("focus", notify);
+        window.removeEventListener("blur", notify);
+      }
+    };
+  }, []);
+
+  return useSyncExternalStore(subscribe, getSnapshot, getSnapshot);
+}