An Electron client for the desktop store

The desktop store put the catalog on ordinary computers, and then asked people to
open a terminal — which on Windows is not even a workable ask, because the
installer is `curl … | sh`. This is the window: a grid of cards, one click to
install a title into the application menu, one to play it, one to remove it.

The CLI stays the product. Every action runs `desktop_store.py --json`, so there
is one catalog logic, one state file and one delete guard; the window never
touches the filesystem itself. That is also why the engine grew `--json` first
rather than this app growing a parser for prose.

It doubles as the Windows install path: with no store on the machine, the app
downloads the engine, the shared core and a config into the same folder the shell
installer would use — Node's https, no curl. An engine older than 1.1.0 cannot be
driven from a window, so the client checks the version and offers to refresh it
instead of failing on the first call.

Deliberate choices worth knowing:

- No renderer framework and no build step. Plain HTML, CSS and JS, one runtime
  dependency. The whole UI is readable in one sitting.
- `contextIsolation` on, `nodeIntegration` off, `sandbox` on, a CSP that permits
  only the app's own script and stylesheet plus images over HTTPS. The renderer
  can do exactly what preload.js exposes and nothing else.
- English and Hungarian, following the system language. The CLIs and the docs stay
  English; this is the one end-user surface where that is not enough.
- `ENGINES` is a list with one entry. The RetroArch store has the same command
  shape, so adding it is an entry, not a rewrite.

Two ways to test it without a working installation in the way: `npm run smoke`
drives the bridge with no window at all, and `npm run uitest` loads the window
once and reports what rendered — the only way a renderer error would otherwise be
noticed, since the main process log stays empty. Both accept a sandbox store
through STORE_ROOT / SMOKE_HOME.

Verified on macOS arm64, including the packaged .app: the store is found, ten
titles list, a sync installs three, and the window renders them as installed with
their Play and Remove buttons. Linux and Windows are unproven, as they are for the
CLI itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

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