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/export.rs

@@ -51,6 +51,13 @@ pub fn sanitize_filename_component(title: &str) -> String {
 /// `~/.config/user-dirs.dirs` (z. B. „Dokumente“), sonst `~/Documents`. Im Flatpak ist nur
 /// dieser Ordner beschreibbar, der Fallback allein träfe bei deutschen Systemen daneben.
 pub fn default_documents_dir() -> PathBuf {
+    // Android: written into the app's own folder first, then copied to Downloads
+    // (`platform::publish_export`)
+    #[cfg(target_os = "android")]
+    if let Some(dir) = crate::platform::android_app().and_then(|app| app.external_data_path()) {
+        return dir.join("Documents");
+    }
+
     if let Ok(dir) = std::env::var("XDG_DOCUMENTS_DIR") {
         if !dir.trim().is_empty() {
             return PathBuf::from(dir);