Record into a Jellyfin library by default; configurable recording location

When no custom directory is set, recordings go to an "SRF Recordings" folder
in the chosen library, or the first TV Shows library, so they show up without
adding a library by hand. The settings page gets a library picker, a check
that the directory is writable, and help for sandboxed systemd services.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Jellyfin.Plugin.SRFPlay/Services/Interfaces/IRecordingService.cs

@@ -75,4 +75,10 @@ public interface IRecordingService
     /// <param name="cancellationToken">The cancellation token.</param>
     /// <returns>A task representing the async operation.</returns>
     Task ProcessRecordingsAsync(CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Resolves where recordings will be written and checks the server can write there.
+    /// </summary>
+    /// <returns>The output directory and the result of a write test.</returns>
+    RecordingOutputInfo GetOutputInfo();
 }