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/Utilities/DisplayTime.cs

@@ -0,0 +1,42 @@
+using System;
+
+namespace Jellyfin.Plugin.SRFPlay.Utilities;
+
+/// <summary>
+/// Converts times into the time zone the plugin writes into text (item names, file names).
+/// SRG sends times with an offset, and DateTime deserialisation turns them into server-local
+/// values, so without this the text follows whatever zone the server happens to run in.
+/// </summary>
+public static class DisplayTime
+{
+    /// <summary>
+    /// Converts a time to the configured display time zone (the server's if none is set).
+    /// </summary>
+    /// <param name="value">The time. Unspecified values are treated as server-local, like deserialised ones.</param>
+    /// <returns>The same instant expressed in the display time zone.</returns>
+    public static DateTime From(DateTime value)
+    {
+        return TimeZoneInfo.ConvertTimeFromUtc(value.ToUniversalTime(), GetZone());
+    }
+
+    /// <summary>
+    /// Gets the current time in the display time zone.
+    /// </summary>
+    /// <returns>The current time in the display time zone.</returns>
+    public static DateTime Now()
+    {
+        return TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, GetZone());
+    }
+
+    private static TimeZoneInfo GetZone()
+    {
+        var id = Plugin.Instance?.Configuration.DisplayTimeZone;
+        if (string.IsNullOrWhiteSpace(id))
+        {
+            return TimeZoneInfo.Local;
+        }
+
+        // An unknown id falls back to the server's zone rather than breaking listings.
+        return TimeZoneInfo.TryFindSystemTimeZoneById(id.Trim(), out var zone) ? zone : TimeZoneInfo.Local;
+    }
+}