feat(native): the app needs TREK 5.1.3 on the server and checks a remembered one once after the update

The fixes that keep the app's sign-in in the app live in the server's UI,
so an updated app on a 5.1.2 server still ended in the browser. The server now
reports nativeApi 2 and the app refuses anything below it, naming 5.1.3.

A server the app remembers from before is not loaded unchecked: the first
launch after the update shows the address screen filled in with it, says why
and checks it right away. A server that is new enough loads again with its
session; an older one gets the update hint.

client/src/native/trekShell.ts

@@ -13,7 +13,12 @@ import type { NativeCapabilities } from './capabilities';
  * worker.
  */
 export interface TrekShellPlugin {
-  getServer(): Promise<{ url: string | null }>;
+  /**
+   * `previousUrl`: the server this app remembered before an update asked for a
+   * newer one (SERVER_CHECK in TrekServerStore). Set only while no server is,
+   * so the address screen can check it again, pre-filled.
+   */
+  getServer(): Promise<{ url: string | null; previousUrl?: string | null }>;
   /**
    * Remembers the server and loads it. The current page is gone afterwards.
    * `signInHosts` are the hosts of a proxy's sign-in page in front of it