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)

Models/AgentRecord.cs

@@ -18,6 +18,8 @@ public class AgentRecord
     public bool UseWebSearch { get; set; }
     public string WebSearchVersion { get; set; } = "";
     public double AvatarScale { get; set; } = 1.0;
+    public double AvatarOffsetX { get; set; }
+    public double AvatarOffsetY { get; set; }
     public string UserId { get; set; }
     public bool IsDeleted { get; set; }
     public string CreatedAt { get; set; }

Models/SessionRecord.cs

@@ -18,6 +18,8 @@ public class SessionRecord
     public string TelegramChatId { get; set; }
     public string TelegramAllowedChatIds { get; set; }
     public double AvatarScale { get; set; } = 1.0;
+    public double AvatarOffsetX { get; set; }
+    public double AvatarOffsetY { get; set; }
     public string ReviewAgentId { get; set; }
     public int MaxReviewIterations { get; set; } = 3;
     public string CreatedAt { get; set; }