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/export.rs

@@ -56,19 +56,16 @@ pub fn default_documents_dir() -> PathBuf {
         }
     }
 
-    if let Ok(home) = std::env::var("HOME") {
-        if !home.trim().is_empty() {
-            let home = PathBuf::from(home);
-            let user_dirs =
-                std::fs::read_to_string(home.join(".config/user-dirs.dirs")).unwrap_or_default();
-            if let Some(dir) =
-                crate::file_picker::parse_user_dir(&user_dirs, "XDG_DOCUMENTS_DIR", &home)
-                    .filter(|dir| *dir != home)
-            {
-                return dir;
-            }
-            return home.join("Documents");
+    if let Some(home) = crate::paths::home_dir() {
+        let user_dirs =
+            std::fs::read_to_string(home.join(".config/user-dirs.dirs")).unwrap_or_default();
+        if let Some(dir) =
+            crate::file_picker::parse_user_dir(&user_dirs, "XDG_DOCUMENTS_DIR", &home)
+                .filter(|dir| *dir != home)
+        {
+            return dir;
         }
+        return home.join("Documents");
     }
 
     PathBuf::from(".")