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>

scripts/publish-gitea-release.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # ==============================================================================
 # Veröffentlicht ein Release auf dem Gitea-Server (gitea.pansi.eu)
-# Lädt .deb, .AppImage, SHA256SUMS.txt und SHA256SUMS.txt.minisig idempotent als Release-Assets hoch.
+# Lädt .deb, .AppImage, .exe, SHA256SUMS.txt und SHA256SUMS.txt.minisig idempotent als Release-Assets hoch.
 # ==============================================================================
 set -euo pipefail
 
@@ -32,6 +32,7 @@ RELEASE_TITLE="ink-notes ${TAG_NAME}"
 
 DEB_FILE="${ROOT_DIR}/target/ink-notes_${VERSION}_amd64.deb"
 APPIMAGE_FILE="${ROOT_DIR}/target/ink-notes_${VERSION}_x86_64.AppImage"
+EXE_FILE="${ROOT_DIR}/target/ink-notes_${VERSION}_x86_64.exe"
 CHECKSUMS_FILE="${ROOT_DIR}/target/SHA256SUMS.txt"
 SIGNATURE_FILE="${CHECKSUMS_FILE}.minisig"
 TEMPLATE_FILE="${ROOT_DIR}/packaging/release-notes.md.tmpl"
@@ -42,7 +43,7 @@ if [ ! -f "${TEMPLATE_FILE}" ]; then
 fi
 
 # Sicherstellen, dass die Artefakte existieren
-if [ ! -f "${DEB_FILE}" ] || [ ! -f "${APPIMAGE_FILE}" ] || [ ! -f "${CHECKSUMS_FILE}" ]; then
+if [ ! -f "${DEB_FILE}" ] || [ ! -f "${APPIMAGE_FILE}" ] || [ ! -f "${EXE_FILE}" ] || [ ! -f "${CHECKSUMS_FILE}" ]; then
     echo "Artefakte nicht vollständig gefunden. Führe build-release.sh aus..."
     "${SCRIPT_DIR}/build-release.sh"
 fi
@@ -236,6 +237,7 @@ except Exception:
 
 upload_asset "${DEB_FILE}"
 upload_asset "${APPIMAGE_FILE}"
+upload_asset "${EXE_FILE}"
 upload_asset "${CHECKSUMS_FILE}"
 upload_asset "${SIGNATURE_FILE}"