fix: calendar room label dedup + room number validation

- BookingCalendar: remove type badge below room number (already shown in group header)
- RoomModal: validate number against existingNumbers before save
  - Red border + error message if number taken
  - Placeholder shows next free number (e.g. if 101,102 exist → placeholder 103)
  - Save button disabled while number is taken

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

src/components/calendar/BookingCalendar.tsx

@@ -437,11 +437,6 @@ export function BookingCalendar({ rooms, bookings, slug, onBookingCreate, onBook
                         <span className="text-sm font-bold text-slate-900 dark:text-slate-100">{room.number}</span>
                         {room.name && !isMobile && <span className="text-xs text-slate-500 dark:text-slate-400">{room.name}</span>}
                       </div>
-                      {!compact && !isMobile && (
-                        <span className={cn('text-xs px-1.5 py-0.5 rounded-md font-medium', getRoomTypeColor(room.type))}>
-                          {room.type}
-                        </span>
-                      )}
                     </div>
                   </div>