feat(update): add nightly Velopack update channel (#80)

Adds `nightly` as a selectable auto-update channel (Windows + macOS).

Client:
- update-manager.ts: add 'nightly' to UpdateChannel (veloChannel already
  yields win-x64-nightly / osx-arm64-nightly, so no logic change)
- main.ts: allow 'nightly' in the runtime IPC channel guard
- preload.ts: add 'nightly' to the preload-local UpdateChannel union
- screen.js / settings.html: add the Nightly option + helper text

CI (nightly.yml):
- derive <pkg>-nightly.<UTC date> version in the setup job
- setup-dotnet (pinned from .build-config.json) so the vpk CLI has net8,
  matching build.yml
- vpk pack win-x64-nightly / osx-arm64-nightly (mac signed + notarized,
  mirroring build.yml's signed/unsigned fallback)
- publish a rolling `nightly` GitHub Release (prerelease=false, latest=false)
  that the in-app updater reads for the nightly channel
- concurrency guard so an overlapping dispatch can't race the rolling release

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main/main.ts

@@ -1108,7 +1108,7 @@ async function startup(): Promise<void> {
         // IPC is untyped at runtime — validate the channel string before forwarding
         // so a renderer bug or compromised page can't pass arbitrary values into
         // the Velopack SDK.
-        const VALID_CHANNELS: readonly string[] = ['stable', 'rc', 'beta', 'alpha'];
+        const VALID_CHANNELS: readonly string[] = ['stable', 'rc', 'beta', 'alpha', 'nightly'];
         if (typeof channel !== 'string' || !VALID_CHANNELS.includes(channel)) {
             return updateManager.getStatus();
         }