feat(native): a proxy's sign-in page in the app has a way out and says what cannot work there

Behind Pangolin or authentik the app shows the proxy's sign-in page itself, and
nothing on it led back to the address screen: a failed passkey left users stuck
on every launch. A bar over that page now offers to change the server and says
that passkeys and security keys do not work there, as a web view cannot use them
for a domain the app is not tied to. It steps aside while the keyboard is up.
On iOS the page also gets the room the keyboard takes, so a button under it can
be scrolled into view. The address screen hands the bar its words in the user's
language when it connects.

client/native-shell/main.ts

@@ -1206,6 +1206,7 @@ async function connected(url: string, signInHosts: string[] = [], welcome?: stri
   await new Promise((r) => setTimeout(r, 700));
   await TrekShell.setServer({
     url,
+    labels: { signInHint: t('native.signInBar.hint'), changeServer: t('native.changeServer') },
     ...(signInHosts.length ? { signInHosts } : {}),
     ...(welcome ? { welcome } : {}),
   });