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/autosave.rs

@@ -690,21 +690,9 @@ impl AutosaveController {
 
 /// Liefert das Verzeichnis für Notfall-Wiederherstellungsdateien ($XDG_DATA_HOME/ink-notes/recovery).
 pub fn recovery_dir() -> PathBuf {
-    if let Ok(xdg) = std::env::var("XDG_DATA_HOME") {
-        if !xdg.is_empty() {
-            return PathBuf::from(xdg).join("ink-notes").join("recovery");
-        }
-    }
-    if let Ok(home) = std::env::var("HOME") {
-        if !home.is_empty() {
-            return PathBuf::from(home)
-                .join(".local")
-                .join("share")
-                .join("ink-notes")
-                .join("recovery");
-        }
-    }
-    PathBuf::from(".").join(".recovery")
+    crate::paths::data_dir()
+        .map(|d| d.join("recovery"))
+        .unwrap_or_else(|| PathBuf::from(".").join(".recovery"))
 }
 
 impl Drop for AutosaveController {