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/i18n/en.rs

@@ -484,4 +484,9 @@ pub const EN: &[(&str, &str)] = &[
     ("✔  Diesen Ordner wählen", "✔  Choose this folder"),
     ("📁  Ändern …", "📁  Change …"),
     ("📁 Speicherort wählen", "📁 Choose storage location"),
+    ("Update installiert – gilt ab dem nächsten Start", "Update installed – takes effect at the next start"),
+    ("🔄 Update installieren", "🔄 Install update"),
+    ("Konnte Backup ({path}) nicht anlegen: {e}. Update abgebrochen, altes Programm unverändert.", "Could not create backup ({path}): {e}. Update cancelled, old program unchanged."),
+    ("Konnte Programm nicht ersetzen: {e}. Altes Programm bleibt unverändert.", "Could not replace the program: {e}. The old program stays unchanged."),
+    ("Ungültiger Programmpfad.", "Invalid program path."),
 ];