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/app.rs

@@ -1664,6 +1664,10 @@ pub fn run_with(
                 window.request_redraw();
             }
             Event::AboutToWait => {
+                #[cfg(target_os = "android")]
+                if crate::android_bridge::has_messages() {
+                    window.request_redraw();
+                }
                 let prev_status = autosave.status();
                 let new_status = autosave.update();
                 if new_status != prev_status {
@@ -4037,6 +4041,13 @@ pub fn run_with(
                         if let Some(requester) = keyboard_requester.as_mut() {
                             raw_input.events.extend(requester.take_deletions());
                         }
+                        // Android: text from the keyboard and files from the system dialog
+                        #[cfg(target_os = "android")]
+                        {
+                            let (events, actions) = crate::android_bridge::take_messages();
+                            raw_input.events.extend(events);
+                            ui_state.queued_actions.extend(actions);
+                        }
                         egui_ctx.begin_frame(raw_input);
 
                         // 2. Auswahl-Overlay rendern (Bounding Box, Griffe, Lasso)