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/SessionEndpoint.cs

@@ -76,8 +76,9 @@ public static class SessionEndpoint
         }
         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;
+        var avatarScale = body.TryGetProperty("avatarScale", out var asc) ? asc.GetDouble() : 1.0;
 
-        var session = await db.CreateSession(userId, name, description, image, agentId, workspace, context, customCommands);
+        var session = await db.CreateSession(userId, name, description, image, agentId, workspace, context, customCommands, avatarScale);
         await ctx.Response.WriteAsJsonAsync(session);
     }