feat(android): pen input, soft keyboard and exports to Downloads (Issues #42, #44)

- Pen vs. finger (#44): the patched winit hands Android pointers with
  tool type stylus/eraser to the app separately (take_pen_samples, with
  hover and button state) instead of as touch events. src/pen.rs turns
  them, like octotablet's events on the desktop, into one PenInput
  stream, so pen buttons, the eraser end and palm rejection work the
  same everywhere. Hover exit right before touch-down does not count as
  leaving; a pen without hover leaves 150 ms after lifting.
- Soft keyboard: shown via AndroidApp::show_soft_input when a text field
  gets focus, hidden when it loses it. The patched winit now fills
  KeyEvent::text, so typed characters reach egui.
- Exports (PDF, PNG, SVG) are written to the app folder and copied to
  Download/ink-notes through the MediaStore (Android 10+, via JNI).

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

Cargo.toml

@@ -73,6 +73,8 @@ wayland-protocols = { version = "0.31", features = ["client", "unstable"] }
 [target.'cfg(target_os = "android")'.dependencies]
 winit = { version = "0.29", features = ["rwh_06", "android-native-activity"] }
 android_logger = { version = "0.13", default-features = false }
+# Saving exports to Downloads through Android's MediaStore
+jni = { version = "0.21", default-features = false }
 log = "0.4"
 
 [package.metadata.android]