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/settings.rs

@@ -258,13 +258,9 @@ impl Default for UiSettings {
 }
 
 impl UiSettings {
-    /// Standardpfad gemäß XDG-Spezifikation.
+    /// Standardpfad gemäß XDG-Spezifikation (see [`crate::paths`] for other systems).
     pub fn default_path() -> Option<PathBuf> {
-        let base = match std::env::var("XDG_CONFIG_HOME") {
-            Ok(dir) if !dir.trim().is_empty() => PathBuf::from(dir),
-            _ => PathBuf::from(std::env::var("HOME").ok()?).join(".config"),
-        };
-        Some(base.join("ink-notes").join("ui.json"))
+        crate::paths::config_dir().map(|d| d.join("ui.json"))
     }
 
     /// Lädt die Einstellungen; bei fehlender oder beschädigter Datei gelten Standardwerte.