Fix latent NRE in Reopen seek and remove dead local

[niksedk]

Summary

Two small fixes found while bug-hunting the recent commits:

1. Latent NRE in the Reopen video-seek flow (MainViewModel.cs). The deferred seek lambda re-read the SelectedSubtitle property instead of the already null-checked local snapshot s:

   var s = SelectedSubtitle;
   if (vp != null && s != null)
   {
       vp.Position = s.StartTime.TotalSeconds;
       Dispatcher.UIThread.Post(() => { vp.Position = SelectedSubtitle.StartTime.TotalSeconds; }); // re-reads property

   Because the Post runs on a later UI tick, closing the file or clearing the selection in that window would make SelectedSubtitle null and throw an NRE on the UI thread. Now uses the captured local s. (The analogous fullscreen path already captures a position local and was unaffected.)

2. Dead local in SubtitleTextInfoHelper.Populate — colorTextTooLong became unused when the CPS background gate was switched to ColorCharactersPerSecond. Removed (CS0219 cleanup).

Testing

• dotnet build src/ui/UI.csproj — succeeds, 0 warnings, 0 errors.

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