fix(android): working signed release APK

- docker/android.Dockerfile: install nodejs (image ships Bun; ci-android-sign.sh
  patches build.gradle with a Node script).
- buffer polyfill: whisper.rn -> safe-buffer requires Node's `buffer`, absent in
  Metro/RN. Add `buffer` dep, alias it in metro.config.js, set the global in
  _layout. Web export unaffected (verified).

Validated locally end-to-end: assembleRelease BUILD SUCCESSFUL, 117MB APK across
all 4 ABIs with librnwhisper.so, signed (CN=Wisp, O=briggen.dev).

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

package.json

@@ -5,6 +5,7 @@
   "dependencies": {
     "@expo/ui": "~56.0.17",
     "@huggingface/transformers": "^4.2.0",
+    "buffer": "^6.0.3",
     "dexie": "^4.4.3",
     "expo": "~56.0.11",
     "expo-constants": "~56.0.18",