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>

src/lib.rs

@@ -22,6 +22,7 @@ pub mod paths;
 pub mod pdf_doc;
 pub mod pdf_merge;
 pub mod pdf_writer;
+pub mod pen;
 pub mod platform;
 pub mod rnote;
 pub mod ruler;
@@ -51,6 +52,7 @@ fn android_main(app: winit::platform::android::activity::AndroidApp) {
     );
     // Panics would otherwise only go to stderr, which Android discards
     std::panic::set_hook(Box::new(|info| log::error!("{info}")));
+    platform::set_android_app(app.clone());
     if let Some(dir) = app.internal_data_path() {
         paths::set_app_dir(dir);
     }