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/ui.rs

@@ -5076,6 +5076,7 @@ pub fn render_ui(
                                 let is_recommended = match updater.runtime() {
                                     crate::updater::RuntimeEnvironment::AppImage { .. } => is_appimage,
                                     crate::updater::RuntimeEnvironment::DebPackage { .. } => is_deb,
+                                    crate::updater::RuntimeEnvironment::WindowsExe { .. } => asset.name.ends_with(".exe"),
                                     _ => false,
                                 };
 
@@ -5141,6 +5142,7 @@ pub fn render_ui(
                                 crate::updater::RuntimeEnvironment::DebPackage { .. } => t!("📦 Paket mit Software-Center öffnen / installieren"),
                                 crate::updater::RuntimeEnvironment::PortableOrDev { .. }
                                 | crate::updater::RuntimeEnvironment::Flatpak => t!("📂 Im Dateimanager anzeigen"),
+                                crate::updater::RuntimeEnvironment::WindowsExe { .. } => t!("🔄 Update installieren"),
                             };
 
                             ui.horizontal(|ui| {