App 2.4.0: pair with the server by QR code

The Server tab gets "Pair with phone" right under the server URL, which
is now pre-filled with https://car.stinnesbeck.com. It opens a dialog
with a QR code and the short code, polls the server, and on approval
saves this car's own token like Save would — nothing to type in the
car. Denied, expired, and pairing switched off on the server are each
reported. PairingClient is tested against a stand-in for the server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/build.gradle

@@ -34,8 +34,8 @@ android {
         applicationId "com.stinnesbeck.nilsmg4"
         minSdk 27
         targetSdk 36
-        versionCode 57
-        versionName "2.3.0"
+        versionCode 58
+        versionName "2.4.0"
 
         // Nothing about the server ships in the APK: its URL and ingest token are entered in
         // the app's Server tab (ServerConfig), and the database is only reachable by the server.