feat: Windows build and Android groundwork (Issues #48, #42)

Shared groundwork:
- paths.rs: folders for data, settings and downloads per system (Linux XDG
  unchanged, Windows %APPDATA%/%LOCALAPPDATA%, Android app folder)
- Linux-only parts behind cfg: Wayland on-screen keyboard (osk/wayland.rs),
  display backend and app id, the vendored winit touch extensions
- run_with(event_loop) so that other entry points can start the app

Windows (#48):
- cross-compiled with MinGW into a portable ink-notes_<version>_x86_64.exe
  (system DLLs only), no console window in release builds
- file picker: user folders and drives; held notebook locks are detected
  (ERROR_LOCK_VIOLATION)
- updater: replaces the running .exe via rename, active at the next start
- octotablet panicking without the Tablet PC components no longer crashes
  the app
- release: .exe built in CI and part of the signed checksums
- all library tests pass under wine

Android (#42):
- android_main entry point, winit with android-native-activity,
  android_logger; octotablet and arboard left out there
- cargo check and the cdylib build for aarch64-linux-android pass; the window
  surface does not follow the activity lifecycle yet

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

src/lib.rs

@@ -18,6 +18,7 @@ pub mod import;
 pub mod library;
 pub mod osk;
 pub mod page_export;
+pub mod paths;
 pub mod pdf_doc;
 pub mod pdf_merge;
 pub mod platform;
@@ -33,3 +34,36 @@ pub mod ui;
 pub mod updater;
 pub mod viewport;
 pub mod xopp;
+
+/// Entry point on Android (Issue #42), called by the native activity.
+///
+/// Builds with `cargo rustc --lib --crate-type cdylib --target aarch64-linux-android`. Not
+/// usable yet: the window surface still has to be created on `Resumed` and dropped on
+/// `Suspended`.
+#[cfg(target_os = "android")]
+#[no_mangle]
+fn android_main(app: winit::platform::android::activity::AndroidApp) {
+    use winit::platform::android::EventLoopBuilderExtAndroid;
+
+    android_logger::init_once(
+        android_logger::Config::default()
+            .with_max_level(log::LevelFilter::Info)
+            .with_tag("ink-notes"),
+    );
+    if let Some(dir) = app.internal_data_path() {
+        paths::set_app_dir(dir);
+    }
+    let event_loop = match winit::event_loop::EventLoopBuilder::new()
+        .with_android_app(app)
+        .build()
+    {
+        Ok(event_loop) => event_loop,
+        Err(err) => {
+            log::error!("event loop: {err}");
+            return;
+        }
+    };
+    if let Err(err) = app::run_with(event_loop) {
+        log::error!("{err}");
+    }
+}