Package the desktop app for Arch, Debian and Windows

electron-builder produces j621-desktop_*_amd64.deb,
j621-desktop-*.pkg.tar.zst (zstd, lean Arch dependencies instead of the
Electron 2 era default set) and a per-user NSIS installer cross-built with
wine. The launcher entry and Electron's desktopName now agree on
io.j621.desktop so window association works, and package metadata points
at the repository homepage and the non-commercial licence.

desktop/README.md

@@ -9,6 +9,9 @@ SPA keeps its normal first-run flow: on launch it asks where the backend is
 The renderer code is not forked: the shell loads `frontend/dist` as built by
 `npm run build` in `frontend/`.
 
+Licensed under the Jake Labs Non-Commercial Software Licence — see
+`../LICENSE`.
+
 ## Layout
 
 ```
@@ -34,17 +37,24 @@ cd desktop && npm run dev
 ## Builds
 
 ```bash
-npm run dist:linux   # release/*.deb + release/*.pkg.tar.zst
-npm run dist:win     # release/J621-Setup-*.exe (cross-built with wine; untested)
+npm run dist:linux   # release/j621-desktop_0.1.0_amd64.deb + j621-desktop-0.1.0.pkg.tar.zst
+npm run dist:win     # release/J621 Setup 0.1.0.exe (cross-built with wine; untested)
 npm run dist:all
 ```
 
 The Arch package can be installed and removed with pacman:
 
 ```bash
-sudo pacman -U release/j621-*.pkg.tar.zst
+sudo pacman -U release/j621-desktop-*.pkg.tar.zst
+sudo pacman -R j621-desktop
 ```
 
+It installs to `/opt/J621` with a `/usr/bin/j621-desktop` symlink and a
+`io.j621.desktop` launcher entry (`StartupWMClass` matches the shell's
+`desktopName`). The Windows installer is a per-user NSIS build, so no admin
+rights are needed to install or update it — but it is unsigned, so SmartScreen
+will warn, and it has not been smoke-tested on real Windows.
+
 ## Smoke test
 
 `npm run smoke` builds everything, runs Electron headless through `xvfb-run`