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>

.gitea/workflows/release.yaml

@@ -35,6 +35,7 @@ jobs:
             libxrandr-dev \
             libxi-dev \
             libxkbcommon-dev \
+            gcc-mingw-w64-x86-64 \
             squashfs-tools \
             dpkg-dev \
             curl \
@@ -62,7 +63,7 @@ jobs:
           cargo clippy --all-targets -- -D warnings
           cargo test
 
-      - name: Build Release Artifacts (.deb, .AppImage, SHA256SUMS.txt + .minisig)
+      - name: Build Release Artifacts (.deb, .AppImage, .exe, SHA256SUMS.txt + .minisig)
         run: |
           chmod +x scripts/*.sh
           ./scripts/build-release.sh