feat: object label per object + {obj:UUID} sentence placeholders

- Annotate: per-object single label input (M2M via db_objects_db_words), auto-save on blur, remove picture-level word section
- Generate: object chips insert {obj:UUID} at cursor position in question/statement textarea
- Live preview resolves {obj:UUID} → actual object label
- PairsList display also resolves placeholders
- Remove F/A/B word chip system from pair form (replaced by object placeholders)
- Backend: POST /api/directus/db-objects/<id>/words replaces existing word with single label

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

frontend/src/api.ts

@@ -463,6 +463,19 @@ export async function getDbObjectWords(objectId: string, token: string): Promise
   return data.data as DbWord[]
 }
 
+export async function saveDbObjectWord(
+  objectId: string,
+  word: { titel_de: string; level: number } | null,
+  token: string
+) {
+  const res = await fetch(`/api/directus/db-objects/${objectId}/words`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json', Authorization: token },
+    body: JSON.stringify(word ? { titel_de: word.titel_de, level: word.level } : { titel_de: '' }),
+  })
+  return res.json()
+}
+
 export async function updateDbPair(
   pairId: string,
   payload: {