ci: build + serve signed Android APK on push to master

- docker/android.Dockerfile: full socket-safe build (JDK17 + SDK + NDK 27 +
  CMake + Bun) -> expo prebuild -> sign (BuildKit --secret) -> assembleRelease.
- ci.yml build-apk job: buildx build, extract APK via docker create/cp, copy
  into the running wisp container so it serves at /wisp.apk; artifact upload.
- app.json: set android.package = dev.briggen.wisp (required by prebuild).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.json

@@ -11,6 +11,7 @@
       "icon": "./assets/expo.icon"
     },
     "android": {
+      "package": "dev.briggen.wisp",
       "adaptiveIcon": {
         "backgroundColor": "#E6F4FE",
         "foregroundImage": "./assets/images/android-icon-foreground.png",