Mobile: standalone signed release APK build (config plugin + script)

- plugins/withReleaseSigning.js — Expo config plugin that injects a release
  signingConfig into android/app/build.gradle at prebuild, reading credentials
  from credentials/keystore.properties (gitignored). Survives CNG regeneration.
- scripts/build-release-apk.sh — generate keystore (once) → prebuild → build →
  install → emit a shareable cardclaws-demo.apk
- .gitignore: exclude /credentials, *.keystore, *.apk (no secrets/binaries)

Produces a standalone APK with the JS bundle embedded — runs with no Metro, no
laptop, no Wi-Fi. Built, signed, installed, and verified running on a physical
device with Metro stopped + the USB route removed.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-mobile/.gitignore

@@ -7,6 +7,11 @@ node_modules/
 /android
 /ios
 
+# Signing secrets + build artifacts — NEVER commit
+/credentials
+*.keystore
+*.apk
+
 # Misc
 *.log
 .DS_Store