feat: HTTP/SOCKS Proxy-Support für alle HttpClient-Requests

- Program.cs: ProxyUrl aus Config lesen, WebProxy auf allen HttpClients via ConfigureHttpClientDefaults
- ServerSettingsEndpoint: ProxyUrl Setting im Frontend (Text-Feld)
- appsettings.json: ProxyUrl Default (leer = kein Proxy)
- Unterstützt HTTP und SOCKS5 Proxies (z.B. socks5://127.0.0.1:1080)
- Betrifft alle ausgehenden Requests: Anthropic, OpenAI, GitHub, Serper, Telegram, Krea

Program.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Net;
 using System.Net.Http;
 using System.Runtime.InteropServices;
 using Gott.Endpoints;
@@ -15,7 +16,20 @@ var builder = WebApplication.CreateBuilder(args);
 if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
     builder.Services.AddWindowsService();
 
-builder.Services.AddHttpClient();
+var proxyUrl = builder.Configuration["ProxyUrl"];
+if (!string.IsNullOrWhiteSpace(proxyUrl))
+{
+    builder.Services.AddHttpClient().ConfigureHttpClientDefaults(b =>
+        b.ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
+        {
+            Proxy = new WebProxy(proxyUrl),
+            UseProxy = true
+        }));
+}
+else
+{
+    builder.Services.AddHttpClient();
+}
 
 var dataDir = Path.Combine(AppContext.BaseDirectory, "data");
 Directory.CreateDirectory(dataDir);