Keep focused agent timelines live and catch up instantly (#2196)

* fix(app): keep focused agent timelines live

Window focus was conflated with app visibility, so switching OS windows could remove the selected timeline subscription. Separate those signals, grace every visibility-driven removal, and cover retained history until authoritative catch-up completes.

* fix(app): keep timeline catch-up failures non-blocking

* fix(app): surface timeline catch-up failures

* fix(app): preserve file preview refocus

* fix(app): preserve optimistic catch-up flow

packages/app/src/utils/app-visibility.test.ts

@@ -0,0 +1,26 @@
+import { expect, test } from "vitest";
+import { isAppActivelyVisible, isAppVisible } from "./app-visibility";
+
+test("a visible desktop app remains visible when another window has focus", () => {
+  const input = {
+    appState: "active",
+    native: false,
+    documentVisible: true,
+    windowFocused: false,
+  };
+
+  expect(isAppVisible(input)).toBe(true);
+  expect(isAppActivelyVisible(input)).toBe(false);
+});
+
+test("a hidden desktop page is neither visible nor actively visible", () => {
+  const input = {
+    appState: "active",
+    native: false,
+    documentVisible: false,
+    windowFocused: true,
+  };
+
+  expect(isAppVisible(input)).toBe(false);
+  expect(isAppActivelyVisible(input)).toBe(false);
+});