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/App.jsx

@@ -4,6 +4,8 @@ import Login from './pages/Login';
 import Dashboard from './pages/Dashboard';
 import DatabaseAdmin from './pages/DatabaseAdmin';
 import TableView from './pages/TableView';
+import ContentHub from './pages/ContentHub';
+import ObjectCreation from './pages/ObjectCreation';
 
 function RequireAuth({ children }) {
   const user = getUser();
@@ -19,6 +21,8 @@ export default function App() {
         <Route path="/" element={<RequireAuth><Dashboard /></RequireAuth>} />
         <Route path="/db" element={<RequireAuth><DatabaseAdmin /></RequireAuth>} />
         <Route path="/db/:tableKey" element={<RequireAuth><TableView /></RequireAuth>} />
+        <Route path="/content" element={<RequireAuth><ContentHub /></RequireAuth>} />
+        <Route path="/content/objects" element={<RequireAuth><ObjectCreation /></RequireAuth>} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>
     </BrowserRouter>