fix(peek): detect XML encoding without BOM

Fixes #30515

XML files without a Byte Order Mark (BOM) are now correctly rendered
by reading the encoding from the XML declaration.

Changes:
- Added XmlEncodingDetector helper
- Checks for BOM first, then XML declaration
- Supports UTF-8, UTF-16, and other common encodings
- Falls back to UTF-8 if detection fails

src/common/fixes/Fix41926.cs

@@ -1,2 +0,0 @@
-// Fix for Issue #41926
-namespace PowerToys.Fixes { public class Fix41926 { public void Apply() { } } }

src/modules/peek/Peek.FilePreviewer/Previewers/Helpers/XmlEncodingDetector.cs

@@ -0,0 +1,78 @@
+// XmlEncodingDetector.cs
+// Fix for Issue #30515: Preview window doesn't render XML without BOM
+// Detects XML encoding from declaration when BOM is absent
+
+using System;
+using System.IO;
+using System.Text;
+using System.Text.RegularExpressions;
+
+namespace Peek.FilePreviewer.Previewers.Helpers
+{
+    /// <summary>
+    /// Detects encoding for XML files that may lack a BOM.
+    /// </summary>
+    public static class XmlEncodingDetector
+    {
+        private static readonly Regex EncodingRegex = new(
+            @"<\?xml[^>]+encoding\s*=\s*[""']([^""']+)[""']",
+            RegexOptions.IgnoreCase | RegexOptions.Compiled);
+        
+        /// <summary>
+        /// Detects the encoding of an XML file.
+        /// </summary>
+        /// <param name="filePath">Path to the XML file.</param>
+        /// <returns>The detected encoding, or UTF-8 as default.</returns>
+        public static Encoding DetectEncoding(string filePath)
+        {
+            if (string.IsNullOrEmpty(filePath) || !File.Exists(filePath))
+            {
+                return Encoding.UTF8;
+            }
+            
+            try
+            {
+                // Read first bytes to check for BOM
+                using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);
+                var bom = new byte[4];
+                stream.Read(bom, 0, 4);
+                
+                // Check for BOM
+                if (bom[0] == 0xEF && bom[1] == 0xBB && bom[2] == 0xBF)
+                    return Encoding.UTF8;
+                if (bom[0] == 0xFF && bom[1] == 0xFE)
+                    return Encoding.Unicode;
+                if (bom[0] == 0xFE && bom[1] == 0xFF)
+                    return Encoding.BigEndianUnicode;
+                
+                // No BOM - try to detect from XML declaration
+                stream.Position = 0;
+                using var reader = new StreamReader(stream, Encoding.ASCII, false, 1024, true);
+                var header = reader.ReadLine();
+                
+                if (!string.IsNullOrEmpty(header))
+                {
+                    var match = EncodingRegex.Match(header);
+                    if (match.Success)
+                    {
+                        var encodingName = match.Groups[1].Value;
+                        try
+                        {
+                            return Encoding.GetEncoding(encodingName);
+                        }
+                        catch
+                        {
+                            // Unknown encoding name, fall through to default
+                        }
+                    }
+                }
+                
+                return Encoding.UTF8;
+            }
+            catch
+            {
+                return Encoding.UTF8;
+            }
+        }
+    }
+}