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/components/ZoneCard.tsx

@@ -42,7 +42,7 @@ export default function ZoneCard({ zone, onClick }: { zone: ZoneStatus; onClick:
   const fanLevel = ac ? (MHI_FAN_SPEEDS as readonly string[]).indexOf(ac.fan_speed || '') + 1 : 0
 
   return (
-    <div className={`card zone-card st-${zone.room_state}`} onClick={onClick}>
+    <div className={`card zone-card st-${zone.room_state}`} onClick={onClick} title={ROOM_STATE_LABELS[zone.room_state]}>
       <div className="zone-card-title">
         <span>
           {zone.name}
@@ -57,7 +57,6 @@ export default function ZoneCard({ zone, onClick }: { zone: ZoneStatus; onClick:
         </span>
         {!zone.auto_mode && <span className="badge badge-outline">Manual</span>}
       </div>
-      <div className={`badge badge-st-${zone.room_state}`}>{ROOM_STATE_LABELS[zone.room_state]}</div>
 
       <div className="zone-card-body">
         <div className="zone-card-main">