v2.0.2 — pairing error surfaced in UI (test build)

Test build. Builds on v2.0.1-mdns-fix-test (cleartext + mDNS service-type
fix). Since that build still showed the same failure, this adds visible
error detail to the pairing-failed dialog instead of a generic message.

What's new

The pairing-failed AlertDialog (shown after "Nearby on this network" or
manual host:port pairing fails) now shows the actual underlying reason
instead of "Could not reach X. Check it's on the same network.":

• If it's a network/platform exception (e.g. cleartext still blocked,
  connection refused, timeout, DNS failure), you'll see the exception
  class + message directly, e.g.
  IOException: Cleartext HTTP traffic to 192.168.1.5 not permitted.
• If the desktop responded but with a non-2xx status, you'll see
  Desktop returned HTTP 404 for /pairing/begin (useful if the endpoint
  doesn't exist yet on the desktop side).
• Also logged via Log.w under tag "CoordinatorApi" if you have logcat
  attached.

How to test

1. Install app-universal-debug.apk (or app-arm64-v8a-debug.apk for a
   smaller download on an arm64 phone).
2. Attempt pairing the same way you did before (mDNS discovery or manual
   host:port).
3. When it fails, tap through and copy/screenshot the exact error text
   in the dialog — that tells us precisely what's still broken.

Debug build, unsigned — allow install from unknown sources / accept the
debug signature warning.