feat(android): follow the activity lifecycle and build an APK (Issue #42)

- The drawing surface is created once the native window exists (Resumed)
  and released when the app goes to the background (Suspended); before
  that everything is saved, as Android may stop the app at any time
- Android's back button acts like Escape instead of closing the app
- Panics are written to logcat; no update check on Android yet
- scripts/package-android.sh builds target/ink-notes_<version>_arm64.apk
  without Gradle (NDK, aapt2, zipalign, apksigner); signing key from
  ANDROID_KEYSTORE/ANDROID_KEYSTORE_PASS/ANDROID_KEY_ALIAS, otherwise
  the debug key

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib.rs

@@ -38,9 +38,7 @@ pub mod xopp;
 
 /// Entry point on Android (Issue #42), called by the native activity.
 ///
-/// Builds with `cargo rustc --lib --crate-type cdylib --target aarch64-linux-android`. Not
-/// usable yet: the window surface still has to be created on `Resumed` and dropped on
-/// `Suspended`.
+/// `scripts/package-android.sh` builds the library and packs it into an APK.
 #[cfg(target_os = "android")]
 #[no_mangle]
 fn android_main(app: winit::platform::android::activity::AndroidApp) {
@@ -51,6 +49,8 @@ fn android_main(app: winit::platform::android::activity::AndroidApp) {
             .with_max_level(log::LevelFilter::Info)
             .with_tag("ink-notes"),
     );
+    // Panics would otherwise only go to stderr, which Android discards
+    std::panic::set_hook(Box::new(|info| log::error!("{info}")));
     if let Some(dir) = app.internal_data_path() {
         paths::set_app_dir(dir);
     }