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/Configuration/PluginConfiguration.cs

@@ -169,6 +169,14 @@ public class PluginConfiguration : BasePluginConfiguration
     /// </summary>
     public string RecordingOutputPath { get; set; } = string.Empty;
 
+    /// <summary>
+    /// Gets or sets the time zone used for times the plugin writes as text: the start time in
+    /// upcoming livestream names and the timestamp in recording file names. An IANA id such as
+    /// "Europe/Zurich"; empty uses the server's time zone. Dates Jellyfin shows itself (e.g.
+    /// PremiereDate) are stored as UTC and follow each viewer's own time zone regardless.
+    /// </summary>
+    public string DisplayTimeZone { get; set; } = string.Empty;
+
     /// <summary>
     /// Gets or sets how many segments back from the live edge livestream playback should start.
     /// Injected as <c>#EXT-X-START:TIME-OFFSET=-(N * targetDuration)</c> into the live media