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>

Cargo.toml

@@ -38,11 +38,9 @@ semver = "1.0"
 sha2 = "0.10"
 ureq = { version = "2.10", default-features = false, features = ["tls", "gzip", "json"] }
 url = "2.5"
-octotablet = "0.1.0"
 
 # Bilder einfügen: Dekodieren (PNG/JPEG/GIF/WebP/BMP) und Systemzwischenablage
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "gif", "webp", "bmp"] }
-arboard = { version = "3", default-features = false, features = ["image-data"] }
 minisign-verify = "0.3.0"
 fs2 = "0.4"
 hayro = "0.8"
@@ -58,11 +56,33 @@ resvg = { version = "0.49", default-features = false, features = ["raster-images
 # Text-Werkzeug: Schriftmetriken und Rasterung (Schriften aus egui)
 ab_glyph = "0.2"
 
+# Pen input with pressure on Linux (Wayland/X11) and Windows (Windows Ink)
+[target.'cfg(not(target_os = "android"))'.dependencies]
+octotablet = "0.1.0"
+# Systemzwischenablage (Bilder und Text)
+arboard = { version = "3", default-features = false, features = ["image-data"] }
+
 # Bildschirmtastatur unter Wayland anfordern (eigenes text-input-Objekt, siehe src/osk.rs)
+[target.'cfg(target_os = "linux")'.dependencies]
 wayland-client = "0.31"
 wayland-backend = { version = "0.3", features = ["client_system"] }
 wayland-protocols = { version = "0.31", features = ["client", "unstable"] }
 
+# Android (Issue #42): winit bound to the native activity. The library is built as
+# cdylib with `cargo rustc --lib --crate-type cdylib --target aarch64-linux-android`
+[target.'cfg(target_os = "android")'.dependencies]
+winit = { version = "0.29", features = ["rwh_06", "android-native-activity"] }
+android_logger = { version = "0.13", default-features = false }
+log = "0.4"
+
+[package.metadata.android]
+package = "eu.pansi.inknotes"
+apk_name = "ink-notes"
+
+[package.metadata.android.sdk]
+min_sdk_version = 26
+target_sdk_version = 34
+
 [profile.dev]
 opt-level = 1 # Erhöht Rendering-Performance auch im Debug-Modus