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

wwwroot/css/form.css

@@ -22,8 +22,8 @@
 }
 
 .avatar-preview {
-  width: 64px;
-  height: 64px;
+  width: 8em;
+  height: 8em;
   background: var(--f2-bg);
   border: var(--border-width) solid var(--f2-border);
   display: flex;
@@ -38,6 +38,32 @@
   width: 100%;
   height: 100%;
   object-fit: cover;
+  transition: scale var(--transition-duration);
+}
+
+.avatar-scale-row {
+  display: flex;
+  align-items: center;
+  gap: var(--badge-spacing);
+  flex: 1;
+  padding: var(--box-padding);
+}
+.avatar-scale-row label {
+  margin: 0;
+}
+.avatar-scale-row input[type="range"] {
+  flex: 1;
+  accent-color: var(--primary);
+}
+.avatar-scale-row span {
+  font-size: 0.85em;
+  min-width: 2.5em;
+  text-align: right;
+  opacity: 0.7;
+}
+
+.avatar-ai-section {
+  width: 100%;
 }
 
 .input-with-btn {