Variable EventsConst

Updater event name constants.

Use these instead of hard-coding string literals when subscribing to updater events from JavaScript:

import { Events, Updater } from "@wailsio/runtime";

Events.On(Updater.Events.UpdateAvailable, (e) => {
    console.log("update found:", e.data.version);
});

Events.On(Updater.Events.DownloadProgress, (e) => {
    const p = e.data;
    console.log(`${p.written} / ${p.total} bytes`);
});

Mirrors the Go-side constants in pkg/updater/events.go and the user-action constants in pkg/updater/window_lifecycle.go. Any changes here must stay in sync with those files — there's an integration test that asserts the strings match.