feat: Statement Creation + objects_created workflow + native language

ObjectCreation:
- Filter: only shows pictures where objects_created=false
- New '✓ Alle Objekte erstellt' button (enabled when ≥1 object linked)
  → PATCHes picture with objects_created:true + auto timestamp
  → removes picture from list immediately

ContentHub:
- Statement Creation tile enabled (was: grayed out)

api.js:
- getUserLang() → reads native_lang from stored user (default 'de')
- langField(suffix) → returns e.g. 'sentence_de' based on user lang
- login() stores native_lang in localStorage user object

StatementCreation (/content/statements):
- Shows pictures where objects_created=true
- Left 1/5: clickable objects list → highlights selections on canvas
- Center 2/5: image with canvas, draws selected object's polygons in color
- Right 2/5: PairsPanel
  - answer_type dropdown + 'Add new pair' toggle
  - PairForm: Question / Positive / Negative textareas
  - HighlightedTextarea: overlay technique, auto-detects words from DB
    (debounced GET /words?titel_de=, colored mark via rgba background)
  - 'Als Wort erstellen' button when text is selected
  - 'Save pair' → creates question + 2 separate statements + pair
    → sentences stored with {{uuid}} placeholders for matched words
    → pair linked to selected object
  - List of existing pairs with question/positive/negative preview

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

src/pages/ContentHub.jsx

@@ -14,9 +14,9 @@ const TOOLS = [
     key: 'statements',
     title: 'Statement Creation',
     icon: '💬',
-    description: 'Aussagen erstellen und mit Wörtern verknüpfen.',
+    description: 'Pairs mit Fragen und Aussagen erstellen, Wörter im Satz markieren.',
     path: '/content/statements',
-    ready: false,
+    ready: true,
   },
   {
     key: 'content',