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/pages/RoomsPage.tsx

@@ -208,6 +208,7 @@ export function RoomsPage() {
         key={editingRoom?.id ?? 'new'}
         open={modalOpen}
         room={editingRoom}
+        existingNumbers={rooms.map(r => r.number)}
         onClose={() => { setModalOpen(false); setSaveError(null) }}
         onSave={handleSave}
         categories={categories}