feat: zuverlässiger Stop-Button für Chat-Streaming

- StreamingTracker: CancellationTokenSource pro Session, Cancel()-Methode
- ChatEndpoint: POST /api/chat/stop Endpoint, linkedCts verknüpft
  Stop-Token mit Client-Disconnect-Token
- AnthropicService: cancellationToken an ExecuteTool durchgereicht
- ToolService: CancellationToken-Parameter, execute-Tool verknüpft
  externen Token mit 180s-Timeout, killt Prozess bei Abbruch
- Frontend: apiStopChat() + stopStreaming() ruft Backend-Stop auf

Services/StreamingTracker.cs

@@ -1,16 +1,43 @@
 using System.Collections.Concurrent;
+using System.Threading;
 
 namespace Gott.Services;
 
 public class StreamingTracker
 {
-    private readonly ConcurrentDictionary<string, string> _streaming = new ConcurrentDictionary<string, string>();
+    private readonly ConcurrentDictionary<string, StreamingState> _streaming = new ConcurrentDictionary<string, StreamingState>();
 
-    public void Start(string sessionId, string chainId) => _streaming[sessionId] = chainId;
+    public CancellationTokenSource Start(string sessionId, string chainId)
+    {
+        var cts = new CancellationTokenSource();
+        _streaming[sessionId] = new StreamingState(chainId, cts);
+        return cts;
+    }
 
-    public void Stop(string sessionId) => _streaming.TryRemove(sessionId, out _);
+    public void Stop(string sessionId)
+    {
+        if (_streaming.TryRemove(sessionId, out var state))
+        {
+            state.Cts.Dispose();
+        }
+    }
+
+    /// <summary>
+    /// Cancels the running stream for a session. Returns true if a stream was cancelled.
+    /// </summary>
+    public bool Cancel(string sessionId)
+    {
+        if (_streaming.TryGetValue(sessionId, out var state))
+        {
+            try { state.Cts.Cancel(); } catch { }
+            return true;
+        }
+        return false;
+    }
 
     public bool IsStreaming(string sessionId) => _streaming.ContainsKey(sessionId);
 
-    public string GetChainId(string sessionId) => _streaming.TryGetValue(sessionId, out var chainId) ? chainId : null;
+    public string GetChainId(string sessionId) => _streaming.TryGetValue(sessionId, out var state) ? state.ChainId : null;
+
+    private record StreamingState(string ChainId, CancellationTokenSource Cts);
 }