Gitea Actions: auto-build signed arm64 APK release on push to main

Mirrors the meditation-timer pipeline; VERSION_CODE/VERSION_NAME are
stamped from the run number via app.config.js so Obtainium sees each
release as an upgrade. Keystore comes from repo secrets.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app.config.js

@@ -0,0 +1,10 @@
+/* Wraps app.json so CI can stamp versions: VERSION_NAME → expo.version,
+   VERSION_CODE → android.versionCode (Obtainium needs it to increase). */
+module.exports = ({ config }) => {
+  const versionCode = parseInt(process.env.VERSION_CODE || '', 10);
+  if (process.env.VERSION_NAME) config.version = process.env.VERSION_NAME;
+  if (Number.isFinite(versionCode)) {
+    config.android = { ...config.android, versionCode };
+  }
+  return config;
+};