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.Cli/Program.cs

@@ -2,5 +2,10 @@ namespace ConvertSRTto3DASS;
 
 internal static class Program
 {
-    static void Main(string[] args) => Cli.Run(args);
+    static void Main(string[] args)
+    {
+        var hadError = Cli.Run(args);
+        if (hadError)
+            Console.ReadLine();
+    }
 }