Adds support for multiple chapters. The previous version ignored any leading chapter numbers in the uri of {chapterId}/images/{image}. It now accounts for that and is working in a locally run user interface for an epub that was broken. I didn't check in the unit test over the GetBookPage method since it requires a specific file. I'll provide that in the PR description, though.

API/Extensions/PathExtensions.cs

@@ -1,4 +1,6 @@
-using System.IO;
+using System;
+using System.Globalization;
+using System.IO;
 
 namespace API.Extensions;
 #nullable enable
@@ -12,4 +14,23 @@ public static class PathExtensions
         if (string.IsNullOrEmpty(extension)) return filepath;
         return Path.GetFullPath(filepath.Replace(extension, string.Empty));
     }
+
+    public static ReadOnlySpan<char> GetFirstSegmentSpan(this string urlKey)
+    {
+        var idx = urlKey.IndexOf('/');
+        return idx < 0 ? urlKey.AsSpan() : urlKey.AsSpan()[..idx];
+    }
+
+    public static ReadOnlySpan<char> GetLastSegmentSpan(this string urlKey)
+    {
+        var idx = urlKey.LastIndexOf('/');
+        return idx < 0 ? urlKey.AsSpan() : urlKey.AsSpan().Slice(idx + 1);
+    }
+
+    public static int? ParseInt(this ReadOnlySpan<char> s)
+    {
+        if (int.TryParse(s, NumberStyles.Any, CultureInfo.InvariantCulture, out var value))
+            return value;
+        return default;
+    }
 }