Add PrefersColorSchemeMiddleware and config setting

- Added `PrefersColorSchemeMiddleware` to handle color scheme headers.
- Updated `Program.cs` to conditionally use the new middleware based on `Experimental:UsePrefersColorSchemeHeader` setting.
- Modified `appsettings.json` to include `Experimental` section with `UsePrefersColorSchemeHeader` setting.

src/Moonglade.Web/Middleware/PrefersColorSchemeMiddleware.cs

@@ -0,0 +1,17 @@
+namespace Moonglade.Web.Middleware;
+
+public class PrefersColorSchemeMiddleware(RequestDelegate next)
+{
+    public async Task InvokeAsync(HttpContext context)
+    {
+        context.Response.OnStarting(() =>
+        {
+            context.Response.Headers["Accept-CH"] = "Sec-CH-Prefers-Color-Scheme";
+            context.Response.Headers["Vary"] = "Sec-CH-Prefers-Color-Scheme";
+            context.Response.Headers["Critical-CH"] = "Sec-CH-Prefers-Color-Scheme";
+            return Task.CompletedTask;
+        });
+
+        await next(context);
+    }
+}

src/Moonglade.Web/Program.cs

@@ -267,6 +267,11 @@ private static void ConfigureMiddleware(WebApplication app, List<CultureInfo> cu
             options.IconFilePath = "/favicon-16x16.png";
         });
 
+        // In v14.11.0, just send the header to the client in order to observe browser behaviour
+        // We will read client's preference in future versions
+        bool usePrefersColorSchemeHeader = app.Configuration.GetSection("Experimental:UsePrefersColorSchemeHeader").Get<bool>();
+        if (usePrefersColorSchemeHeader) app.UseMiddleware<PrefersColorSchemeMiddleware>();
+
         app.UseMiddleware<PoweredByMiddleware>();
         app.UseMiddleware<DNTMiddleware>();
 

src/Moonglade.Web/appsettings.json

@@ -73,6 +73,9 @@
     "SetLastModifiedHeader": true,
     "CacheMinutes": 20
   },
+  "Experimental": {
+    "UsePrefersColorSchemeHeader": true
+  },
   "Logging": {
     "LogLevel": {
       "Default": "Warning",