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>

public/index.html

@@ -1273,6 +1273,7 @@
 
                     <div class="admin-content-section">
                         <h4>Bestehende VOs</h4>
+                        <p style="color:#666; font-size:13px;">Die mit ★ markierte VO wird bei der Schaltungserfassung automatisch vorausgewählt.</p>
                         <div class="table-container">
                             <table class="admin-table">
                                 <thead>
@@ -1281,6 +1282,7 @@
                                         <th>Nummer</th>
                                         <th>Bezeichnung</th>
                                         <th>Aktiv</th>
+                                        <th>Standard</th>
                                         <th>Aktionen</th>
                                     </tr>
                                 </thead>