Only offer vacant rooms in the tenant-invite room dropdown

The "Mieter einladen" form previously listed every room regardless of
occupancy, letting a landlord accidentally assign a new invite to an
already-occupied room. Filters lastRooms to status === 'VACANT' for
that select only — ticket/inventory room pickers still show all rooms
since those aren't about assigning a tenancy.

web-dashboard/index.html

@@ -1351,7 +1351,9 @@
   function populateRoomSelect() {
     const select = document.getElementById('inviteRoom');
     select.innerHTML = '<option value="">– kein Zimmer zuordnen –</option>';
-    lastRooms.forEach(r => {
+    // Nur freie Zimmer zur Auswahl anbieten — ein bereits belegtes Zimmer
+    // kann nicht zusätzlich vergeben werden.
+    lastRooms.filter(r => r.status === 'VACANT').forEach(r => {
       const opt = document.createElement('option');
       opt.value = r.id;
       opt.textContent = r.roomNumber;