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/main.rs

@@ -3,6 +3,14 @@
 //!
 //! Desktop entry point; the app itself is [`ink_notes::app`].
 
+// No console window next to the app on Windows (debug builds keep it for the log)
+#![cfg_attr(all(windows, not(debug_assertions)), windows_subsystem = "windows")]
+
+#[cfg(not(target_os = "android"))]
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     ink_notes::app::run()
 }
+
+/// On Android the app starts in `android_main` of the library.
+#[cfg(target_os = "android")]
+fn main() {}