Dashboard: drop room-state badge, colour left border to match NewBook status colours

- Remove status badge from ZoneCard, keep left-border indicator only (with hover title for accessibility)
- Rename heating_up label from 'Heating Up' to 'Preparing'
- Recolour room states: booked=green, occupied=blue, cooling_down=purple (matches NewBook's status colours); vacant stays grey, heating_up stays amber
- Decouple rad-icon.heating from --st-occupied (now blue) to --st-heating-up (amber) so an actively-heating radiator icon still reads warm

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/types.ts

@@ -8,7 +8,7 @@ export type PhotoType = 'device' | 'serial_plate'
 export const ROOM_STATE_LABELS: Record<RoomState, string> = {
   vacant: 'Vacant',
   booked: 'Booked',
-  heating_up: 'Heating Up',
+  heating_up: 'Preparing',
   occupied: 'Occupied',
   cooling_down: 'Cooling Down',
 }