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/platform.rs

@@ -82,6 +82,51 @@ pub fn desktop_event_loop() -> Result<winit::event_loop::EventLoop<()>, winit::e
     builder.build()
 }
 
+/// Waits until the window can be drawn to. On Android the native window only exists after
+/// `Event::Resumed` (Issue #42); on the desktop it is there right away.
+pub fn wait_until_resumed(
+    #[allow(unused_variables)] event_loop: &mut winit::event_loop::EventLoop<()>,
+) {
+    #[cfg(target_os = "android")]
+    {
+        use winit::event::Event;
+        use winit::platform::pump_events::{EventLoopExtPumpEvents, PumpStatus};
+        let mut resumed = false;
+        while !resumed {
+            let status = event_loop.pump_events(None, |event, _| {
+                if matches!(event, Event::Resumed) {
+                    resumed = true;
+                }
+            });
+            if let PumpStatus::Exit(_) = status {
+                std::process::exit(0);
+            }
+        }
+    }
+}
+
+/// Runs the event loop until the app quits. On Android the loop was already started by
+/// [`wait_until_resumed`], so it is driven with `pump_events` (`run` would refuse to start).
+pub fn run_event_loop<F>(
+    #[allow(unused_mut)] mut event_loop: winit::event_loop::EventLoop<()>,
+    #[allow(unused_mut)] mut handler: F,
+) -> Result<(), winit::error::EventLoopError>
+where
+    F: FnMut(winit::event::Event<()>, &winit::event_loop::EventLoopWindowTarget<()>),
+{
+    #[cfg(target_os = "android")]
+    {
+        use winit::platform::pump_events::{EventLoopExtPumpEvents, PumpStatus};
+        loop {
+            if let PumpStatus::Exit(_) = event_loop.pump_events(None, &mut handler) {
+                return Ok(());
+            }
+        }
+    }
+    #[cfg(not(target_os = "android"))]
+    event_loop.run(handler)
+}
+
 /// Gives the window the app id (Wayland: app_id, X11: WM_CLASS), so that dock and task bar
 /// show the app icon for it.
 pub fn with_app_id(