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)

wwwroot/css/form.css

@@ -107,23 +107,28 @@
   gap: var(--badge-spacing);
 }
 
-.av-scale-row {
+.av-scale-row,
+.av-offset-row {
   display: flex;
   align-items: center;
   gap: var(--badge-spacing);
 }
 
-.av-scale-row label {
+.av-scale-row label,
+.av-offset-row label {
   margin: 0;
   white-space: nowrap;
+  min-width: 2.5em;
 }
 
-.av-scale-row input[type="range"] {
+.av-scale-row input[type="range"],
+.av-offset-row input[type="range"] {
   flex: 1;
   accent-color: var(--primary);
 }
 
-.av-scale-row .av-scale-value {
+.av-scale-row .av-scale-value,
+.av-offset-row span[class*="-value"] {
   font-size: 0.85em;
   min-width: 2.5em;
   text-align: right;