Android: sign release builds with our own upload key

A config plugin adds a release signing config that reads the key from
MUSICSHARE_UPLOAD_* in ~/.gradle/gradle.properties (kept out of the repo),
and falls back to the debug key where that isn't set up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/mobile/app.json

@@ -50,7 +50,8 @@
           "cameraPermission": "musicshare uses the camera to scan friends' QR codes.",
           "microphonePermission": false
         }
-      ]
+      ],
+      "./plugins/withReleaseSigning"
     ],
     "experiments": {
       "reactCompiler": true