fix: Agent bricht bei Page-Refresh nicht mehr ab + SSE-Writes crash-sicher

Ursache: WriteSseEvent in ProcessStream warf OperationCanceledException
bei Client-Disconnect (Page-Refresh) → sseCt war gelinkt mit dem
ASP.NET cancellationToken → Exception wurde im catch-Block als
'stopped' behandelt → Agent brach ab obwohl nur der Browser refreshed wurde.

Fixes:
- ProcessStream: Alle WriteSseEvent durch TryWriteSse ersetzt
  SSE-Writes bei Client-Disconnect werden jetzt zu stillen No-Ops
  statt den gesamten Agent-Flow per Exception zu killen
- catch-Blöcke: OperationCanceledException nur bei workCt/hardCt als
  'stopped' behandeln, neuer catch für Client-Disconnect als
  'client_disconnected' (Fallback, sollte nicht mehr auftreten)
- done-Event nach Stop: sseCt → CancellationToken.None damit das
  done-Event trotz gecancelltem sseCt noch gesendet wird
- Frontend: done-Handler rendert Chain-Badge direkt im apiGetChain-
  Callback statt Race-Condition mit finally-Block
- Frontend: 'stopped' Badge wieder angezeigt (war nur fälschlich bei
  Page-Refresh, jetzt korrekt nur bei echtem Stop)

Services/AnthropicService.cs

@@ -272,10 +272,25 @@ public class AnthropicService
             await TryWriteSse(response, "error", new { error = "Max iterations erreicht" }, sseCt);
             await TryWriteSse(response, "done", new { chainId = chain.ChainId }, sseCt);
         }
-        catch (OperationCanceledException)
+        catch (OperationCanceledException) when (workCt.IsCancellationRequested || hardCt.IsCancellationRequested)
         {
             // Stop-Button gedrückt (workCt oder hardCt wurde gecancelled)
             await FinalizeChain(chain, usage, iterations, "stopped", sw);
+            // sseCt ist hier bereits cancelled (gelinkt mit workCt), daher CancellationToken.None
+            // Der SSE-Stream zum Client ist noch offen – nur der Stop-Button hat ausgelöst
+            await TryWriteSse(response, "done", new
+            {
+                model = modelId,
+                inputTokens = usage.InputTokens,
+                outputTokens = usage.OutputTokens,
+                cacheCreationTokens = usage.CacheCreationTokens,
+                cacheReadTokens = usage.CacheReadTokens,
+                iterations,
+                chainId,
+                durationMs = sw.ElapsedMilliseconds,
+                stopReason = "stopped",
+                planMode
+            }, CancellationToken.None);
         }
         catch (Exception) when (workCt.IsCancellationRequested || hardCt.IsCancellationRequested)
         {
@@ -283,6 +298,27 @@ public class AnthropicService
             // ObjectDisposedException/IOException aus statt OperationCanceledException.
             // Wenn workCt/hardCt gecancelled ist, ist das ein regulärer Stop.
             await FinalizeChain(chain, usage, iterations, "stopped", sw);
+            // sseCt ist hier bereits cancelled (gelinkt mit workCt), daher CancellationToken.None
+            await TryWriteSse(response, "done", new
+            {
+                model = modelId,
+                inputTokens = usage.InputTokens,
+                outputTokens = usage.OutputTokens,
+                cacheCreationTokens = usage.CacheCreationTokens,
+                cacheReadTokens = usage.CacheReadTokens,
+                iterations,
+                chainId,
+                durationMs = sw.ElapsedMilliseconds,
+                stopReason = "stopped",
+                planMode
+            }, CancellationToken.None);
+        }
+        catch (OperationCanceledException)
+        {
+            // Client-Disconnect (cancellationToken gecancelled, aber NICHT workCt/hardCt)
+            // Agent-Arbeit ist leider schon abgebrochen – Chain als "client_disconnected" finalisieren
+            // (sollte durch TryWriteSse eigentlich nicht mehr passieren)
+            await FinalizeChain(chain, usage, iterations, "client_disconnected", sw);
         }
         catch (Exception ex)
         {
@@ -1136,7 +1172,7 @@ public class AnthropicService
                     if (cbType == "text")
                     {
                         currentTextBlock.Clear();
-                        await WriteSseEvent(response, "text_start", new { index = idx }, sseCt);
+                        await TryWriteSse(response, "text_start", new { index = idx }, sseCt);
                     }
                     else if (cbType == "tool_use")
                     {
@@ -1148,7 +1184,7 @@ public class AnthropicService
                         currentToolId = cb.GetProperty("id").GetString() ?? "";
                         currentToolName = cb.GetProperty("name").GetString() ?? "";
                         currentToolInputJson.Clear();
-                        await WriteSseEvent(response, "tool_start", new { name = currentToolName, id = currentToolId }, sseCt);
+                        await TryWriteSse(response, "tool_start", new { name = currentToolName, id = currentToolId }, sseCt);
                     }
                     else if (cbType == "server_tool_use")
                     {
@@ -1160,7 +1196,7 @@ public class AnthropicService
                         currentToolId = cb.GetProperty("id").GetString() ?? "";
                         currentToolName = cb.GetProperty("name").GetString() ?? "";
                         currentToolInputJson.Clear();
-                        await WriteSseEvent(response, "web_search_start", new { name = currentToolName, id = currentToolId }, sseCt);
+                        await TryWriteSse(response, "web_search_start", new { name = currentToolName, id = currentToolId }, sseCt);
                     }
                     else if (cbType == "web_search_tool_result")
                     {
@@ -1185,7 +1221,7 @@ public class AnthropicService
                     {
                         var text = delta.GetProperty("text").GetString() ?? "";
                         currentTextBlock.Append(text);
-                        await WriteSseEvent(response, "text_delta", new { text }, sseCt);
+                        await TryWriteSse(response, "text_delta", new { text }, sseCt);
                     }
                     else if (deltaType == "input_json_delta")
                     {
@@ -1206,7 +1242,7 @@ public class AnthropicService
                             if (text.Length > 0)
                             {
                                 assistantContent.Add(new { type = "text", text });
-                                await WriteSseEvent(response, "text_end", new { }, sseCt);
+                                await TryWriteSse(response, "text_end", new { }, sseCt);
                             }
                             currentTextBlock.Clear();
                         }
@@ -1243,7 +1279,7 @@ public class AnthropicService
                                     }
                                 }
 
-                                await WriteSseEvent(response, "user_choice", new
+                                await TryWriteSse(response, "user_choice", new
                                 {
                                     toolCallId = currentToolId,
                                     question,
@@ -1268,7 +1304,7 @@ public class AnthropicService
                             workCt.ThrowIfCancellationRequested();
                             executedTools.Add(new ToolResult(currentToolId, toolResult, isError));
 
-                            await WriteSseEvent(response, "tool_end", new
+                            await TryWriteSse(response, "tool_end", new
                             {
                                 name = currentToolName,
                                 input = currentToolInputJson.ToString(),
@@ -1296,7 +1332,7 @@ public class AnthropicService
                             // Web-Search-Ergebnis von Anthropic – als Raw-JSON in assistantContent aufnehmen
                             assistantContent.Add(webSearchResultBlocks[idx]);
                             var sources = webSearchSources.TryGetValue(idx, out var s) ? s : new List<object>();
-                            await WriteSseEvent(response, "web_search_end", new { id = currentToolId, sources }, sseCt);
+                            await TryWriteSse(response, "web_search_end", new { id = currentToolId, sources }, sseCt);
                         }
                         else if (bt.EndsWith("_tool_result") && serverToolResultBlocks.TryGetValue(idx, out var srvResultBlock))
                         {