bbox/polygon durch selections ersetzen

- bbox und polygon Felder in Directus versteckt (Daten bleiben)
- Alle Auswahlen laufen nur noch über das selections-Feld
- CanvasObject, DirectusObject, API und Zeichenlogik umgestellt
- Objekte mit mehreren Auswahlen werden korrekt auf Canvas gezeichnet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/DrawIt.tsx

@@ -50,8 +50,7 @@ export default function DrawIt() {
   const canvasObjects: CanvasObject[] = objects.map((obj, i) => ({
     id: obj.id,
     visible: obj.visible !== false,
-    bbox: obj.bbox,
-    polygon: obj.polygon,
+    selections: obj.selections,
     index: i + 1,
     hierarchy: 1,
   }))
@@ -88,11 +87,8 @@ export default function DrawIt() {
     if (!currentPicture || !token || currentSelections.length === 0) return
     setSaving(true)
     try {
-      const first = currentSelections[0]
       const obj = await createDirectusObject({
         picture: currentPicture.id,
-        bbox: first.mode === 'rect' ? (first.bbox ?? null) : null,
-        polygon: first.mode === 'polygon' ? (first.polygon ?? null) : null,
         selections: currentSelections,
         user_notes: userNotes.trim() || null,
         parent: parentId,
@@ -182,7 +178,7 @@ export default function DrawIt() {
                       />
                       <div className="object-item-text">
                         <strong>Objekt {i + 1}</strong>
-                        <span>{obj.bbox ? `Rect ${Math.round(obj.bbox.width)}×${Math.round(obj.bbox.height)}` : obj.polygon ? `Poly ${obj.polygon.length} Pkt.` : '–'}</span>
+                        <span>{obj.selections?.length ? `${obj.selections.length} Auswahl(en)` : '–'}</span>
                         {obj.parent && <span style={{ color: 'var(--primary-muted-fg)', fontSize: 11 }}>↳ Kind von #{objects.findIndex(o => o.id === obj.parent) + 1}</span>}
                       </div>
                       <button