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>

src/app/_layout.tsx

@@ -1,3 +1,10 @@
+// Polyfill the Buffer global early — some native deps (whisper.rn -> safe-buffer)
+// expect it. Harmless on web, where Buffer simply isn't used.
+import { Buffer } from 'buffer';
+if (typeof (globalThis as { Buffer?: unknown }).Buffer === 'undefined') {
+  (globalThis as { Buffer?: unknown }).Buffer = Buffer;
+}
+
 import { DarkTheme, DefaultTheme, Stack, ThemeProvider } from 'expo-router';
 import { StatusBar } from 'expo-status-bar';
 import { useColorScheme } from 'react-native';