enrolment carries the CA, and one store holds it on every platform

An APK built in this VM pins this VM's CA, so it can never reach the
host's ai-server -- which is exactly the iris Android client's situation
(cross-compiled here, run against the host). So ai-server now puts the CA
in every enrollment link it mints, base64url of its DER under the 'ca'
parameter wg-app-link just learned to add, and client_core parses it back
out as PEM. Nothing has to be built on the machine it talks to.

Refused rather than ignored where 'ca' does not decode: a link that named
a certificate and then pinned nothing is the one outcome nothing
downstream could notice.

EnrollmentStore moves out of desktop-app into client_core::config, since
the Android client needs the same file for the same reason and only the
directory differs by platform (AGENTS.md's sharing rule). desktop-app's
--ca becomes the override for a link that carried none.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

android-shell/Cargo.lock

@@ -82,6 +82,7 @@ checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 name = "client-core"
 version = "0.1.0"
 dependencies = [
+ "base64",
  "event-model",
  "log",
  "pulldown-cmark",