feat: Content Verwaltung — ContentHub + Object Creation

Adds a full Content Management section:
- Nav link "Content" in header (all pages)
- ContentHub at /content with 3 tiles (Object/Statement/Content Creation)
- ObjectCreation at /content/objects:
  - Top bar: ← → pagination through all "uploaded" pictures
  - Left panel (1/5): existing objects per picture with their words, + button to start draw mode
  - Center: image on dark bg with canvas overlay for polygon drawing
  - Right words panel (1/5): picture words + new object words (each with search/create)
  - Right toolbar (1/5): draw instructions, "Auswahl hinzufügen", numbered selections list, "Objekt speichern" (requires ≥1 selection + ≥1 object word)
- Canvas drawing: click=add point, dblclick=close polygon, live preview line to mouse cursor
- Selections stored as [{points:[{x,y},...]}] (relative 0-1 coords) in objects.selections JSONB
- Object saved with status "draft", linked picture + words

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

src/pages/ContentHub.jsx

@@ -0,0 +1,64 @@
+import { useNavigate } from 'react-router-dom';
+import Layout from '../components/Layout';
+
+const TOOLS = [
+  {
+    key: 'objects',
+    title: 'Object Creation',
+    icon: '🖼️',
+    description: 'Bilder durchblättern, Bereiche einzeichnen und Objekte mit Wörtern verknüpfen.',
+    path: '/content/objects',
+    ready: true,
+  },
+  {
+    key: 'statements',
+    title: 'Statement Creation',
+    icon: '💬',
+    description: 'Aussagen erstellen und mit Wörtern verknüpfen.',
+    path: '/content/statements',
+    ready: false,
+  },
+  {
+    key: 'content',
+    title: 'Content Creation',
+    icon: '✨',
+    description: 'Inhalte zusammenstellen und veröffentlichen.',
+    path: '/content/creation',
+    ready: false,
+  },
+];
+
+export default function ContentHub() {
+  const navigate = useNavigate();
+
+  return (
+    <Layout back="/">
+      <div className="max-w-3xl mx-auto">
+        <h1 className="text-2xl font-bold text-slate-800 mb-2">Content Verwaltung</h1>
+        <p className="text-slate-500 mb-8">Wähle ein Tool um loszulegen.</p>
+        <div className="grid grid-cols-1 sm:grid-cols-3 gap-5">
+          {TOOLS.map(tool => (
+            <button
+              key={tool.key}
+              onClick={() => tool.ready && navigate(tool.path)}
+              className={`text-left rounded-2xl p-6 shadow-sm border transition-all
+                ${tool.ready
+                  ? 'bg-white border-slate-200 hover:shadow-md hover:border-indigo-300 cursor-pointer'
+                  : 'bg-slate-50 border-slate-100 opacity-50 cursor-not-allowed'
+                }`}
+            >
+              <div className="text-3xl mb-3">{tool.icon}</div>
+              <h2 className="font-semibold text-slate-800 mb-1">{tool.title}</h2>
+              <p className="text-sm text-slate-500">{tool.description}</p>
+              {!tool.ready && (
+                <span className="inline-block mt-3 text-xs bg-slate-200 text-slate-500 rounded-full px-2 py-0.5">
+                  Bald verfügbar
+                </span>
+              )}
+            </button>
+          ))}
+        </div>
+      </div>
+    </Layout>
+  );
+}