feat: Avatar Position-Offset (translate X/Y)

Backend:
- AgentRecord + SessionRecord: AvatarOffsetX, AvatarOffsetY Properties
- DB-Migration: avatar_offset_x/y REAL DEFAULT 0 auf agents + sessions
- Agent/Session Endpoints: Create + Update lesen/schreiben Offset-Felder
- GetSessionsWithStats: Offset-Felder ans Frontend

Frontend:
- Avatar-Editor: X/Y Offset-Slider (-50% bis +50%) im Modal
  - buildTransformStyle() Helper für transform: scale() translate()
  - getOffsetX()/getOffsetY() Public API
  - Reset bei Clear/AI-Generate/Upload
- ui.js: buildAgentCard + buildSessionCard rendern transform mit Offset
- Agent-Page + Session-Page: initialOffsetX/Y + Save
- CSS: .av-offset-row Styles (gruppiert mit .av-scale-row)

Endpoints/AgentEndpoint.cs

@@ -65,7 +65,9 @@ public static class AgentEndpoint
             UseMemory = !body.TryGetProperty("useMemory", out var um) || um.GetBoolean(),
             UseWebSearch = body.TryGetProperty("webSearchVersion", out var wsv) && !string.IsNullOrEmpty(wsv.GetString()),
             WebSearchVersion = body.TryGetProperty("webSearchVersion", out var wsv2) ? wsv2.GetString() ?? "" : "",
-            AvatarScale = body.TryGetProperty("avatarScale", out var asc) ? asc.GetDouble() : 1.0
+            AvatarScale = body.TryGetProperty("avatarScale", out var asc) ? asc.GetDouble() : 1.0,
+            AvatarOffsetX = body.TryGetProperty("avatarOffsetX", out var aox) ? aox.GetDouble() : 0,
+            AvatarOffsetY = body.TryGetProperty("avatarOffsetY", out var aoy) ? aoy.GetDouble() : 0
         };
 
         agent = await db.CreateAgent(agent);
@@ -102,6 +104,8 @@ public static class AgentEndpoint
             agent.UseWebSearch = !string.IsNullOrEmpty(agent.WebSearchVersion);
         }
         if (body.TryGetProperty("avatarScale", out var asc)) agent.AvatarScale = asc.GetDouble();
+        if (body.TryGetProperty("avatarOffsetX", out var aox)) agent.AvatarOffsetX = aox.GetDouble();
+        if (body.TryGetProperty("avatarOffsetY", out var aoy)) agent.AvatarOffsetY = aoy.GetDouble();
 
         await db.UpdateAgent(agent);
         memoryService.InitializeAgentFiles(agentId, agent.Name);