feat: Avatar Scale (0.5×–3×) + Krea AI Bildgenerierung für Avatare

Backend:
- SessionRecord + AgentRecord: neues Feld AvatarScale (double, Default 1.0)
- DatabaseService: avatar_scale Migration, in Create/Update/GetWithStats
- SessionEndpoint + AgentEndpoint: avatarScale in Create/Update
- api.js: apiKreaStatus(), apiKreaGenerate()

Frontend:
- Session + Agent Templates: Range-Slider (0.5×–3×) in Avatar-Section
- Krea AI Generate: Prompt-Input + Magic-Button (nur wenn KreaToken konfiguriert)
- Live-Preview: transform: scale() auf Avatar-Vorschau
- Dashboard: scale() auf Session- und Agent-Card-Images
- form.css: avatar-scale-row + avatar-ai-section Styling

Models/AgentRecord.cs

@@ -17,6 +17,7 @@ public class AgentRecord
     public bool UseMemory { get; set; } = true;
     public bool UseWebSearch { get; set; }
     public string WebSearchVersion { get; set; } = "";
+    public double AvatarScale { get; set; } = 1.0;
     public string UserId { get; set; }
     public bool IsDeleted { get; set; }
     public string CreatedAt { get; set; }

Models/SessionRecord.cs

@@ -16,6 +16,7 @@ public class SessionRecord
     public string TelegramBotToken { get; set; }
     public string TelegramWebhookSecret { get; set; }
     public string TelegramChatId { get; set; }
+    public double AvatarScale { get; set; } = 1.0;
     public string CreatedAt { get; set; }
     public string UpdatedAt { get; set; }
 }