feat: DefaultPath als Base-Path für User-Workspaces

- WorkspaceHelper.cs: Zentrale Workspace-Auflösung (relativ → absolut)
- Session-Workspace wird relativ zum User-DefaultPath gespeichert
- System-Prompt bekommt aufgelösten absoluten Pfad
- Path-Traversal-Schutz: '..' in Workspace-Pfaden blockiert
- Browse-API gibt '..' nicht mehr als Eintrag zurück
- Folder-Browser: basePath-Limit, Back-Button navigiert zum Parent
- DefaultPath nur durch Admin bearbeitbar (Profil: readonly)
- Legacy: Absolute Pfade in session.Workspace weiterhin kompatibel

Endpoints/SessionEndpoint.cs

@@ -63,7 +63,17 @@ public static class SessionEndpoint
         var imageRaw = body.TryGetProperty("image", out var img) ? img.GetString() : null;
         var image = imageService.ResizeDataUrl(imageRaw);
         var agentId = body.TryGetProperty("agentId", out var a) ? a.GetString() : null;
-        var workspace = body.TryGetProperty("workspace", out var w) ? w.GetString() : null;
+        var workspaceRaw = body.TryGetProperty("workspace", out var w) ? w.GetString() : null;
+        // Relativen Workspace validieren (Path-Traversal verhindern)
+        var workspace = !string.IsNullOrWhiteSpace(workspaceRaw) && !System.IO.Path.IsPathRooted(workspaceRaw)
+            ? WorkspaceHelper.ValidateRelativeWorkspace(workspaceRaw)
+            : workspaceRaw;
+        if (!string.IsNullOrWhiteSpace(workspaceRaw) && workspace == null)
+        {
+            ctx.Response.StatusCode = 400;
+            await ctx.Response.WriteAsJsonAsync(new { error = "Ungültiger Workspace-Pfad (.. nicht erlaubt)" });
+            return;
+        }
         var context = body.TryGetProperty("context", out var ctxEl) ? ctxEl.GetString() : null;
         var customCommands = body.TryGetProperty("customCommands", out var cc) && cc.ValueKind != JsonValueKind.Null ? cc.GetRawText() : null;
 
@@ -89,6 +99,23 @@ public static class SessionEndpoint
         }
 
         var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
+
+        // Workspace-Pfad validieren (Path-Traversal verhindern)
+        if (body.TryGetProperty("workspace", out var wsEl))
+        {
+            var wsRaw = wsEl.GetString();
+            if (!string.IsNullOrWhiteSpace(wsRaw) && !System.IO.Path.IsPathRooted(wsRaw))
+            {
+                var validated = WorkspaceHelper.ValidateRelativeWorkspace(wsRaw);
+                if (validated == null)
+                {
+                    ctx.Response.StatusCode = 400;
+                    await ctx.Response.WriteAsJsonAsync(new { error = "Ungültiger Workspace-Pfad (.. nicht erlaubt)" });
+                    return;
+                }
+            }
+        }
+
         await db.UpdateSession(sessionId, body, imageService);
 
         var session = await db.GetSession(sessionId);
@@ -141,7 +168,10 @@ public static class SessionEndpoint
         var agentMemoryDir = memoryService.GetAgentMemoryDir(agent.AgentId);
         var agentSkills = await db.GetAgentSkills(agent.AgentId);
         var user = await db.GetUser(userId);
-        var systemPrompt = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, agentSkills, skillService, userPersona: user?.GetEffectivePersona());
+        var resolvedWorkspace = WorkspaceHelper.ResolveWorkspace(session, user);
+        if (string.IsNullOrEmpty(resolvedWorkspace) && agentSkills.Any())
+            resolvedWorkspace = "/tmp";
+        var systemPrompt = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, agentSkills, skillService, userPersona: user?.GetEffectivePersona(), resolvedWorkspace: resolvedWorkspace);
 
         ctx.Response.ContentType = "text/plain; charset=utf-8";
         await ctx.Response.WriteAsync(systemPrompt);
@@ -175,7 +205,10 @@ public static class SessionEndpoint
         var agentMemoryDir = memoryService.GetAgentMemoryDir(agent.AgentId);
         var agentSkills = await db.GetAgentSkills(agent.AgentId);
         var user = await db.GetUser(userId);
-        var blocks = SystemPromptBuilder.BuildBlocks(agent, session, markedMessages, memoryService, agentMemoryDir, agentSkills, skillService, userPersona: user?.GetEffectivePersona());
+        var resolvedWorkspace = WorkspaceHelper.ResolveWorkspace(session, user);
+        if (string.IsNullOrEmpty(resolvedWorkspace) && agentSkills.Any())
+            resolvedWorkspace = "/tmp";
+        var blocks = SystemPromptBuilder.BuildBlocks(agent, session, markedMessages, memoryService, agentMemoryDir, agentSkills, skillService, userPersona: user?.GetEffectivePersona(), resolvedWorkspace: resolvedWorkspace);
 
         await ctx.Response.WriteAsJsonAsync(blocks.Select(b => new { b.Icon, b.Label, b.Content }));
     }
@@ -230,7 +263,9 @@ public static class SessionEndpoint
             return;
         }
 
-        var workingDir = !string.IsNullOrWhiteSpace(session.Workspace) ? session.Workspace : "/tmp";
+        var user = await db.GetUser(userId);
+        var resolvedWs = WorkspaceHelper.ResolveWorkspace(session, user);
+        var workingDir = !string.IsNullOrWhiteSpace(resolvedWs) ? resolvedWs : "/tmp";
 
         if (mode == "foreground")
         {
@@ -343,7 +378,8 @@ public static class SessionEndpoint
             return;
         }
 
-        var workingDir = !string.IsNullOrWhiteSpace(session.Workspace) ? session.Workspace : null;
+        var user = await db.GetUser(userId);
+        var workingDir = WorkspaceHelper.ResolveWorkspace(session, user);
         if (string.IsNullOrEmpty(workingDir))
         {
             ctx.Response.StatusCode = 400;