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>

docs/FILE_FORMAT.de.md

@@ -29,6 +29,8 @@ Dieses Dokument beschreibt, wo und wie ink-notes Notizbücher speichert, und die
 ~/.config/ink-notes/ui.json            ($XDG_CONFIG_HOME) Einstellungen der Oberfläche
 ```
 
+Unter Windows liegt dieselbe Struktur in `%APPDATA%\ink-notes` (Notizbücher, `recovery`, `ui.json`); heruntergeladene Updates landen in `%LOCALAPPDATA%\ink-notes`.
+
 Der Ordner `notebooks/` lässt sich in den Einstellungen verlegen (`storage_dir` in `ui.json`), z. B. nach `~/Nextcloud/ink-notes`; `recovery/` bleibt, wo es ist.
 
 - **Notizbuch-IDs** werden aus dem Titel und dem Zeitpunkt gebildet (z. B. `mathematik_1775460000`); das erste Notizbuch heißt `default`.