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/osk/wayland.rs

@@ -92,6 +92,10 @@ impl KeyboardRequester {
         }
     }
 
+    /// The text field lost focus. Nothing to do: the compositor hides the keyboard once
+    /// winit disables text input.
+    pub fn release(&mut self) {}
+
     /// Tastendrücke für das seit dem letzten Aufruf gewünschte Löschen (vor `begin_frame`
     /// aufrufen und an die egui-Eingabe anhängen)
     pub fn take_deletions(&mut self) -> Vec<egui::Event> {