Replace VO edit prompt() with a proper modal dialog

The browser's native prompt() chain was awkward and made it
impossible to clear the bezeichnung in a single, predictable
flow. The new modal mirrors the user-management dialog: it shows
the provider read-only, lets you edit nummer and bezeichnung,
and exposes the aktiv flag explicitly. Empty bezeichnung now
correctly clears the field.

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

public/index.html

@@ -1552,6 +1552,35 @@
             </div>
         </div>
 
+        <!-- Modal: VO bearbeiten -->
+        <div id="voEditModal" style="display:none; position:fixed; inset:0; background:rgba(0,0,0,0.5); z-index:1000;">
+            <div style="position:absolute; top:50%; left:50%; transform:translate(-50%,-50%); background:white; border-radius:8px; width:90%; max-width:480px; padding:24px; box-shadow:0 10px 30px rgba(0,0,0,0.2);">
+                <h3 style="margin-top:0;">VO bearbeiten</h3>
+                <input type="hidden" id="voEditId">
+                <div class="form-group" style="margin-bottom:12px;">
+                    <label>Anbieter</label>
+                    <input type="text" id="voEditAnbieter" readonly style="width:100%; padding:8px; border:1px solid #ccc; border-radius:4px; background:#f5f5f5;">
+                </div>
+                <div class="form-group" style="margin-bottom:12px;">
+                    <label for="voEditNummer">VO-Nummer</label>
+                    <input type="text" id="voEditNummer" style="width:100%; padding:8px; border:1px solid #ccc; border-radius:4px;">
+                </div>
+                <div class="form-group" style="margin-bottom:12px;">
+                    <label for="voEditBezeichnung">Bezeichnung</label>
+                    <input type="text" id="voEditBezeichnung" placeholder="z.B. Fachhandel (leer = keine)" style="width:100%; padding:8px; border:1px solid #ccc; border-radius:4px;">
+                </div>
+                <div class="form-group" style="margin-bottom:18px;">
+                    <label style="display:flex; align-items:center; gap:8px; cursor:pointer;">
+                        <input type="checkbox" id="voEditAktiv"> Aktiv
+                    </label>
+                </div>
+                <div style="text-align:right;">
+                    <button type="button" onclick="closeVoEditModal()" style="margin-right:8px;">Abbrechen</button>
+                    <button type="button" class="btn-success" onclick="saveVoFromModal()">Speichern</button>
+                </div>
+            </div>
+        </div>
+
         <div id="messages"></div>
     </div>