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>

client-core/Cargo.toml

@@ -37,6 +37,10 @@ ureq = { version = "3", features = ["json"] }
 # the same parser at the same version, rather than a hand-written splitter
 # that would drift from it.
 pulldown-cmark = "0.13.4"
+# The enrollment link's `ca` parameter is base64url of the CA's DER
+# (`config::parse_link`). Same version `wg-app-link` already pins for the
+# minting half, so a workspace that has both resolves one copy.
+base64 = "0.23"
 # The logging facade only -- `log_ring` implements a `log::Log` backend and
 # wraps whichever real one the platform installed (`android_logger` on the
 # phone, `env_logger` on the desktop), which is why neither of those is a