Per-Anbieter default VO with auto-select in the schaltung wizard

- New nullable anbieter.default_vo_id column (migration adds it to
  existing DBs); deleting a VO that some provider points at clears
  the reference automatically.
- GET /api/anbieter now includes default_vo_id + default_vo_nummer
  + default_vo_bezeichnung so the wizard can preselect without an
  extra round-trip.
- New PUT /api/anbieter/:id/default-vo endpoint. Validates the VO
  belongs to the given provider; accepts vo_id=null to clear.
- VO admin table grew a "Standard" column and an "Als Standard" /
  "Standard entfernen" button on each row.
- When the user picks a provider in step 1 of the wizard, the VO
  dropdown now preselects the default VO (falling back to the only
  VO if there is exactly one), and immediately loads the
  matching tariffs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

routes/anbieter.js

@@ -5,10 +5,32 @@ const asyncHandler = require('../middleware/asyncHandler');
 const router = express.Router();
 
 router.get('/', asyncHandler(async (req, res) => {
-    const rows = await db.all('SELECT * FROM anbieter ORDER BY name');
+    const rows = await db.all(
+        `SELECT a.*, v.nummer AS default_vo_nummer, v.bezeichnung AS default_vo_bezeichnung
+         FROM anbieter a
+         LEFT JOIN vos v ON a.default_vo_id = v.id
+         ORDER BY a.name`
+    );
     res.json(rows);
 }));
 
+// Set or clear the default VO for a provider. Pass vo_id=null to clear.
+router.put('/:id/default-vo', asyncHandler(async (req, res) => {
+    const { id } = req.params;
+    const { vo_id } = req.body || {};
+    const anbieter = await db.get('SELECT id FROM anbieter WHERE id = ?', [id]);
+    if (!anbieter) return res.status(404).json({ error: 'Anbieter nicht gefunden' });
+    if (vo_id) {
+        const vo = await db.get('SELECT anbieter_id FROM vos WHERE id = ?', [vo_id]);
+        if (!vo) return res.status(400).json({ error: 'VO nicht gefunden' });
+        if (vo.anbieter_id !== Number(id)) {
+            return res.status(400).json({ error: 'VO gehört nicht zu diesem Anbieter' });
+        }
+    }
+    await db.run('UPDATE anbieter SET default_vo_id = ? WHERE id = ?', [vo_id || null, id]);
+    res.json({ message: 'Standard-VO aktualisiert', default_vo_id: vo_id || null });
+}));
+
 router.post('/', asyncHandler(async (req, res) => {
     const { name } = req.body || {};
     if (!name || !name.trim()) {