refactor: merge nav — rename to Inserate, remove Neues Inserat sidebar entry

New listings are created via the button on the Inserate page, not as a
first-class nav destination.

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

src/pages/supply/MyListings.tsx

@@ -71,7 +71,7 @@ export default function MyListings() {
     <Box sx={{ maxWidth: 960, mx: 'auto', px: 3, py: 4 }}>
       <Box sx={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', mb: 3 }}>
         <Box>
-          <Typography variant="h5" sx={{ fontWeight: 700 }}>Meine Inserate</Typography>
+          <Typography variant="h5" sx={{ fontWeight: 700 }}>Inserate</Typography>
           <Typography variant="body2" color="text.secondary">
             Direkt erstellte Inserate — unabhängig vom Portfolio
           </Typography>