Windows installer: cross-build on Linux and publish to a downloads folder

scripts/windows-publish.sh builds the NSIS setup.exe for
x86_64-pc-windows-msvc through cargo-xwin, versioned like the RPM and
APK, and copies it into a folder to serve: the newest three installers,
a stable mtgc-latest-x64-setup.exe, and SHA256SUMS.

README.md

@@ -138,6 +138,45 @@ From then on, `sudo dnf upgrade` picks up new releases.
 - The RPM is built against this machine's libraries, so it installs on this
   Fedora release and newer.
 
+### Distributing to Windows
+
+The Windows installer is cross-built here on Linux and published to a plain
+downloads folder you host. `scripts/windows-publish.sh` builds it through
+[cargo-xwin](https://github.com/rust-cross/cargo-xwin) and needs, once:
+
+```bash
+sudo dnf install mingw-nsis-base llvm lld clang
+```
+
+```bash
+cargo install --locked cargo-xwin
+```
+
+```bash
+rustup target add x86_64-pc-windows-msvc
+```
+
+Then, for each release:
+
+```bash
+./scripts/windows-publish.sh ~/win-mtgc     # or: npm run windows:publish -- ~/win-mtgc
+```
+
+On Windows, download and run the setup.exe. A newer one upgrades the
+installed app in place.
+
+- **Serve the whole directory.** It holds the versioned installers
+  (`mtgc_0.1.<n>_x64-setup.exe`, the newest three), `mtgc-latest-x64-setup.exe`
+  as a stable link to the newest, and `SHA256SUMS`.
+- Only the NSIS `setup.exe` can be built off Windows; an MSI needs WiX, which
+  runs only on Windows. The first build downloads the MSVC CRT and Windows SDK
+  into `~/.cache/cargo-xwin`.
+- The installer isn't code-signed, so SmartScreen warns about an unknown
+  publisher the first time: **More info → Run anyway**. It fetches WebView2 if
+  the machine lacks it (Windows 10 and 11 normally ship it).
+- Versions work as on Android: `0.1.<commits on HEAD>`, see
+  [Distributing to phones](#distributing-to-phones-f-droid).
+
 ## Using it
 
 - **Add Cards** — type a name (typos OK); ↑/↓ then Enter adds a copy.