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

Endpoints/AgentEndpoint.cs

@@ -64,7 +64,8 @@ public static class AgentEndpoint
             UseUserInfo = !body.TryGetProperty("useUserInfo", out var uu) || uu.GetBoolean(),
             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() ?? "" : ""
+            WebSearchVersion = body.TryGetProperty("webSearchVersion", out var wsv2) ? wsv2.GetString() ?? "" : "",
+            AvatarScale = body.TryGetProperty("avatarScale", out var asc) ? asc.GetDouble() : 1.0
         };
 
         agent = await db.CreateAgent(agent);
@@ -100,6 +101,7 @@ public static class AgentEndpoint
             agent.WebSearchVersion = wsv.GetString() ?? "";
             agent.UseWebSearch = !string.IsNullOrEmpty(agent.WebSearchVersion);
         }
+        if (body.TryGetProperty("avatarScale", out var asc)) agent.AvatarScale = asc.GetDouble();
 
         await db.UpdateAgent(agent);
         memoryService.InitializeAgentFiles(agentId, agent.Name);