feat(mobile): finalize XPREM OTA config with real app id and certificate

Server registration is complete: app id 9a66f3c2-fda7-416f-87e2-6db833b32e9d,
real code-signing certificate (public half), staging/production channels
mapped server-side. Drops the app-id placeholder guard (cert guard stays) and
re-includes certificate.pem past the root *.pem ignore rule, which had been
silently excluding it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/mobile/certs/.gitignore

@@ -1,3 +1,6 @@
 # The update-signing private key belongs to the XPREM server, never to the app.
 private-key.pem
 *.key
+# The public certificate must be committed despite the root *.pem rule: the
+# expo-updates plugin embeds it into every build.
+!certificate.pem