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R1: Umbau auf Multi-Projekt-Struktur (Vorbild PolytraderSharp)

Kurskorrektur: Vorbild ist PolytraderSharp (C#), nicht die veraltete Python-Version.
Reiner Strukturumbau, Verhalten unveraendert.

- src/IBKRTrader.Core (classlib): Core-Code + UI-Contract (ModuleFormBase, WindowManager)
- src/IBKRTrader.Modules.CongressTrading (classlib, referenziert nur Core)
- Root: IBKRTrader.App (WinExe) referenziert Core + Modul
- tests/IBKRTrader.Tests: Referenzen auf Core + Modul
- Neue .slnx; ungenutztes HtmlAgilityPack entfernt
- docs/ARCHITECTURE.md: korrigierter Ziel-Stand + R-Phasenplan
- Build + 38/38 Tests gruen; App startet

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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src/IBKRTrader.Core/Settings/SettingsService.cs

@@ -0,0 +1,49 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace IBKRTrader.Core.Settings;
+
+/// <summary>
+/// Lädt und speichert settings.json neben der EXE.
+/// Singleton – wird beim App-Start einmalig geladen.
+/// </summary>
+public class SettingsService
+{
+    private static readonly string SettingsPath =
+        Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "settings.json");
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented              = true,
+        DefaultIgnoreCondition     = JsonIgnoreCondition.Never,
+        PropertyNameCaseInsensitive = true
+    };
+
+    public AppSettings Settings { get; private set; } = new();
+
+    public void Load()
+    {
+        if (!File.Exists(SettingsPath))
+        {
+            Save(); // Defaults auf Disk schreiben
+            return;
+        }
+
+        try
+        {
+            var json = File.ReadAllText(SettingsPath);
+            Settings = JsonSerializer.Deserialize<AppSettings>(json, JsonOptions) ?? new AppSettings();
+        }
+        catch
+        {
+            Settings = new AppSettings();
+            Save();
+        }
+    }
+
+    public void Save()
+    {
+        var json = JsonSerializer.Serialize(Settings, JsonOptions);
+        File.WriteAllText(SettingsPath, json);
+    }
+}