feat(updater): nightly builds and a stable/nightly update channel (#1608)

* 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>

* fix(updater): compute the nightly version once and keep re-runs safe

Review follow-ups: the nightly version is resolved by a leading job and
handed to every build job, and the patch is bumped only when the base
tag already exists so the release-cut window stays below the imminent
release. A re-run never deletes a published nightly; only a draft left
by a failed run is replaced. Typed update-status literals in the
remaining specs carry the new channel fields.

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

* test(packaging): expect the nightly-version prerequisite in the build workflow graph

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

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/settings.interface.ts

@@ -1,3 +1,4 @@
+import type { AppUpdateChannel } from './app-update-channel.util';
 import { Language } from './language.enum';
 import { StreamFormat } from './stream-format.enum';
 import { Theme } from './theme.enum';
@@ -189,6 +190,15 @@ export interface Settings {
      * value means `'normal'`. F11 leaves or re-enters fullscreen.
      */
     startupWindowMode?: StartupWindowMode;
+    /**
+     * Desktop update channel: `stable` (tagged releases) or `nightly`
+     * (prereleases published from every master merge). Electron only;
+     * a missing value means `stable`. Mirrored into the main-process config
+     * on Save so the startup update check already uses it. Forward-only:
+     * switching back to stable keeps the installed nightly until a newer
+     * stable release exists.
+     */
+    updateChannel?: AppUpdateChannel;
     /** Show the desktop footer bar for external playback status */
     showExternalPlaybackBar?: boolean;
     /** Strip country/group prefixes like "US | " or "UK - " from channel names */