Show times in a configurable time zone

SRG sends times with an offset (e.g. 13:55+02:00). They are stored as
DateTime, which deserialisation converts to server-local time, and two
places then wrote that straight into text: the "[dd.MM HH:mm]" prefix
on upcoming livestream names and the timestamp in recording file names
(DateTime.Now). On a server running in UTC both were two hours behind
Swiss time, e.g. a qualifying starting at 13:55 was listed as 11:55.

A new "Display Time Zone" setting (IANA id, e.g. Europe/Zurich) now
decides the zone for both; empty keeps the server's zone, and an
unknown id falls back to it rather than breaking the listing.
Utilities/DisplayTime does the conversion.

Also fixes PremiereDate on upcoming livestreams, which was set to that
server-local value although Jellyfin expects UTC. It was only correct
on UTC servers; it is now converted, like the other channel items.

Verified on simulated UTC and Europe/Paris servers: the same instant
gives 11:55 / 13:55 / 07:55 for server zone / Europe/Zurich /
America/New_York, and PremiereDate is 11:55 UTC on both.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Jellyfin.Plugin.SRFPlay/Services/RecordingService.cs

@@ -451,7 +451,7 @@ public class RecordingService : IRecordingService, IDisposable
 
             // Build output file path
             var safeTitle = SanitizeFileName(entry.Title);
-            var timestamp = DateTime.Now.ToString("yyyy-MM-dd_HHmm", CultureInfo.InvariantCulture);
+            var timestamp = DisplayTime.Now().ToString("yyyy-MM-dd_HHmm", CultureInfo.InvariantCulture);
             var outputPath = Path.Combine(GetRecordingOutputPath(), $"{safeTitle}_{timestamp}.mkv");
             entry.OutputPath = outputPath;