Fix file path encoding edge cases in HTML export (#1351)

DiscordChatExporter.Core/Utils/Url.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Text;
+
+namespace DiscordChatExporter.Core.Utils;
+
+public static class Url
+{
+    public static string EncodeFilePath(string filePath)
+    {
+        var buffer = new StringBuilder();
+        var position = 0;
+
+        while (true)
+        {
+            if (position >= filePath.Length)
+                break;
+
+            var separatorIndex = filePath.IndexOfAny([':', '/', '\\'], position);
+            if (separatorIndex < 0)
+            {
+                buffer.Append(Uri.EscapeDataString(filePath[position..]));
+                break;
+            }
+
+            // Append the segment
+            buffer.Append(Uri.EscapeDataString(filePath[position..separatorIndex]));
+
+            // Append the separator
+            buffer.Append(
+                filePath[separatorIndex] switch
+                {
+                    // Normalize slashes
+                    '\\' => '/',
+                    var c => c,
+                }
+            );
+
+            position = separatorIndex + 1;
+        }
+
+        return buffer.ToString();
+    }
+}