feat: Payload-Preview im Chat-Menü + Reminder von hinten gezählt

- Neuer Endpoint GET /api/sessions/{id}/payload-preview
  Gibt den exakten Anthropic-API-Payload als JSON zurück (read-only)
- Chat-Menü: 'Payload anzeigen' öffnet Modal mit formatiertem JSON
- Reminder-Logik umgebaut: halbes Intervall (15) vor Ende,
  dann volle Intervalle (30) rückwärts statt von vorne gezählt
- CSS: Payload-Modal Styling, kleine Variable-Fixes

Endpoints/SessionEndpoint.cs

@@ -26,6 +26,7 @@ public static class SessionEndpoint
         app.MapDelete("/api/sessions/{sessionId}", HandleDelete);
         app.MapGet("/api/sessions/{sessionId}/system-prompt", HandleSystemPrompt);
         app.MapGet("/api/sessions/{sessionId}/system-prompt-blocks", HandleSystemPromptBlocks);
+        app.MapGet("/api/sessions/{sessionId}/payload-preview", HandlePayloadPreview);
         app.MapGet("/api/sessions/{sessionId}/streaming", HandleStreaming);
         app.MapPost("/api/sessions/{sessionId}/exec", HandleExec);
         app.MapGet("/api/sessions/{sessionId}/git-status", HandleGitStatus);
@@ -266,6 +267,75 @@ public static class SessionEndpoint
         await ctx.Response.WriteAsJsonAsync(blocks.Select(b => new { b.Icon, b.Label, b.Content }));
     }
 
+    private static async Task HandlePayloadPreview(HttpContext ctx, DatabaseService db, MemoryService memoryService, SkillService skillService, ChatService chatService, ToolService toolService, ServerSettingsService settings)
+    {
+        var result = await ResolvePromptContext(ctx, db, memoryService);
+        if (result == null) return;
+        var (session, agent, marked, memDir, skills, user, workspace, planMode) = result.Value;
+
+        // System-Prompt
+        var promptBlocks = SystemPromptBuilder.BuildBlocks(agent, session, marked, memoryService, memDir,
+            skills, skillService, userPersona: user?.GetEffectivePersona(), resolvedWorkspace: workspace, planMode: planMode);
+        var systemPromptText = string.Join("\n\n", promptBlocks.Select(b => b.Content));
+
+        var systemPrompt = new List<object>();
+        if (!string.IsNullOrWhiteSpace(systemPromptText))
+            systemPrompt.Add(new { type = "text", text = systemPromptText });
+
+        // Model
+        var modelId = ChatService.ResolveModelId(agent.Model);
+
+        // Messages aus der DB (exakt wie beim Chat)
+        var messages = await chatService.BuildHistoryFromDb(session.SessionId, promptBlocks: promptBlocks);
+
+        // Tools (exakt wie AddToolsToPayload es machen würde)
+        var hasWorkspace = !string.IsNullOrEmpty(workspace);
+        var includeMemory = agent.UseMemory && !string.IsNullOrEmpty(agent.AgentId);
+        var includeGitHubSearch = agent.UseGitHubSearch && toolService.HasGitHubToken;
+        var includeSerperSearch = agent.UseSerperSearch && toolService.HasSerperKey;
+        var hasTools = hasWorkspace || includeMemory || includeGitHubSearch || includeSerperSearch;
+
+        var tools = new List<object>();
+        if (hasTools)
+        {
+            var allTools = toolService.GetToolDefinitions(
+                includeFileTools: hasWorkspace,
+                includeMemory: includeMemory,
+                includeUserChoice: agent.AllowUserChoice,
+                includeGitHubSearch: includeGitHubSearch,
+                includeSerperSearch: includeSerperSearch);
+            if (planMode)
+                allTools = toolService.FilterReadOnlyTools(allTools);
+            tools.AddRange(allTools);
+        }
+        if (!string.IsNullOrEmpty(agent.WebSearchVersion))
+            tools.Add(new { type = agent.WebSearchVersion, name = "web_search", max_uses = 5 });
+
+        var toolChoice = hasTools || !string.IsNullOrEmpty(agent.WebSearchVersion) ? "any" : null;
+        var effectiveChoice = toolChoice == "any" ? "any" : "auto";
+
+        // Payload zusammenbauen (Anthropic-Format)
+        var payload = new Dictionary<string, object>
+        {
+            ["model"] = modelId,
+            ["max_tokens"] = 16384,
+            ["messages"] = messages,
+            ["stream"] = true
+        };
+
+        if (systemPrompt.Count > 0)
+            payload["system"] = systemPrompt;
+
+        if (tools.Count > 0)
+        {
+            payload["tools"] = tools;
+            payload["tool_choice"] = new { type = effectiveChoice };
+        }
+
+        ctx.Response.ContentType = "application/json; charset=utf-8";
+        await ctx.Response.WriteAsync(JsonSerializer.Serialize(payload, new JsonSerializerOptions { WriteIndented = true }));
+    }
+
     private static async Task HandleStreaming(HttpContext ctx, DatabaseService db, StreamingTracker streamingTracker)
     {
         var userId = ctx.GetUserId();