Android app (TWA) in android/, built with podman; serve assetlinks.json for it

README.md

@@ -163,6 +163,28 @@ rsvg-convert -w 512 -h 512 icon-maskable.svg -o icon-maskable-512.png
 rsvg-convert -w 180 -h 180 icon-maskable.svg -o apple-touch-icon.png
 ```
 
+### Android app
+
+`android/` is the site as an Android app: a Trusted Web Activity, which opens the public site in
+the phone's Chrome, full screen, with no code of its own (Google's `androidbrowserhelper` does it
+all). Build it with podman:
+
+```
+android/build.sh
+```
+
+That builds an image with JDK 17, the Android SDK and Gradle, and writes a signed
+`android/out/simpleadmin-web.apk`. The signing key is kept outside the repo, in
+`~/.config/simpleadmin-web/android/` (`release.jks` and its `password`), and made on the first run.
+Back it up: an APK signed with another key won't install over the old one. Then:
+
+- Set `SAW_ANDROID_PACKAGE` and `SAW_ANDROID_CERT_SHA256` (the fingerprint `build.sh` prints) so
+  the panel serves `/.well-known/assetlinks.json`. Without it, Chrome shows its URL bar over the app.
+- Bump `versionCode` in `android/app/build.gradle` for every APK you hand out.
+- The site and start page are at the top of `android/app/build.gradle`; never change `applicationId`.
+
+The app needs Chrome (or another browser with TWA support) on the phone.
+
 ## Limitations and caveats
 
 - **Tested against a real MySQL 8.4** with SimpleAdmin 1.8.2b's migrations applied, and a fake game