fix: wire up GUI browse buttons, make text boxes editable, pause CLI on error, register codepage provider

GUI:
- Attach Click event handlers to browse input/output buttons (were defined but never wired up)
- Remove ReadOnly=true from input/output text boxes so users can type or copy paths

CLI:
- Make Cli.Run return bool to signal errors
- Only pause with Console.ReadLine() when an error occurs (not on success)
- Fix default case to return true

Core:
- Add System.Text.Encoding.CodePages package for CP1252 SRT file support
- Call Encoding.RegisterProvider(CodePagesEncodingProvider.Instance) before reading files

ConvertSRTto3DASS.Core/SrtConverter.cs

@@ -16,6 +16,9 @@ public static class SrtConverter
 
     public static ConversionResult Convert(ConversionOptions options)
     {
+        // Register legacy codepage support (needed for CP1252, etc.)
+        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+
         var srtText = ReadSrt(options.InputPath);
         var subtitles = ParseSrt(srtText);
         var header = CreateHeader(options);