feat(android): Java activity for keyboard text and the system file dialog (Issue #42)

InkActivity extends NativeActivity:
- An invisible view with its own input connection receives the soft
  keyboard's text (commitText, deleteSurroundingText), so characters
  without a key event (umlauts by long press, ...) arrive as well. The
  field is a visible-password field, so keyboards commit each character
  instead of composing words.
- Import and "insert image" open the system file dialog
  (ACTION_OPEN_DOCUMENT); the chosen file is copied to the cache folder
  and handed to Rust by path.

src/android_bridge.rs calls the activity through JNI and receives its
native calls; package-android.sh compiles the class with javac and d8.

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

src/ui.rs

@@ -690,6 +690,11 @@ impl UiState {
     fn open_file_picker(&mut self, kind: PickerKind) {
         let dir = self.last_picker_dir.clone();
         self.close_overlays();
+        // Android: the app can't look into other apps' folders; the system dialog can
+        #[cfg(target_os = "android")]
+        if crate::android_bridge::pick_file(kind) {
+            return;
+        }
         self.file_picker = Some(FilePicker::open(kind, dir.as_deref()));
     }