feat(updater): nightly builds and a stable/nightly update channel

Every master push publishes its artifacts as a prerelease of
4gray/iptvnator-nightly instead of the rolling test-master draft, with a
version of <next patch>-nightly.<commit date>.<run number> applied in
every build job. Settings → About gains an Update channel switch;
AppUpdateService re-points electron-updater per check (feed repository,
allowPrerelease, channel name, allowDowngrade reset) and reads release
notes from the repository the requested version belongs to. Channel
switches are forward-only: a nightly build stays until a newer stable
release exists.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

apps/web/src/app/settings/settings.component.html

@@ -92,6 +92,8 @@
                 @case ('about') {
                     <app-settings-about-section
                         [isDesktop]="isDesktop"
+                        [form]="settingsForm"
+                        [updateChannelOptions]="updateChannelOptions"
                         [version]="appUpdate.version()"
                         [buildCommit]="buildCommit"
                         [updateMessage]="appUpdate.updateMessage()"