Add file:// protocol to absolute asset paths in HTML exports (#1475)

* Initial plan

* Add file:// protocol prefix to absolute paths in HTML exports

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Improve absolute path detection for cross-platform compatibility

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Remove unused System.IO import

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Use Path.IsPathFullyQualified for absolute path detection (fixes #1473)

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Core/Utils/Url.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Text;
 
 namespace DiscordChatExporter.Core.Utils;
@@ -10,6 +11,19 @@ public static class Url
         var buffer = new StringBuilder();
         var position = 0;
 
+        // For absolute paths, prepend file:// protocol for proper browser handling
+        if (Path.IsPathFullyQualified(filePath))
+        {
+            buffer.Append("file://");
+
+            // On Windows, we need to add an extra slash before the drive letter
+            // e.g., file:///C:/path instead of file://C:/path
+            if (!filePath.StartsWith('/') && !filePath.StartsWith('\\'))
+            {
+                buffer.Append('/');
+            }
+        }
+
         while (true)
         {
             if (position >= filePath.Length)