Fix: vacant card after a departure no longer spans left past the checkout

bookingsForSiteOnDate uses strict `departure > viewDate`, so a booking
that departed yesterday (departure_date = yesterday) doesn't appear as
prevBooking when computing today's card. prevVacant was therefore true,
causing the vacant card to span left into yesterday's margin — visually
indistinguishable from an ongoing stay.

Fix: check departingBookingForSite(yesterday) as prevDepartedBooking.
Include it in prevVacant guard (prevVacant = !prevBooking && !prevDepartedBooking)
and use its status for previous_status so the correct coloured bracket
appears on the left without the card spanning into yesterday.

Result: day after a checkout shows vacant card with coloured left bracket
(blue = still-arrived, purple = departed in NewBook) but full left border;
consecutive vacant days from the second night onward still span left into
each other correctly.

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

backend/src/lib/booking-flow.js

@@ -79,6 +79,13 @@ export function classifyRoom(site, allBookings, viewDate, yesterday, tomorrow, o
   const prevBooking = bookingsForSiteOnDate(allBookings, siteId, yesterday)[0] ?? null
   const nextBooking = bookingsForSiteOnDate(allBookings, siteId, tomorrow)[0]  ?? null
 
+  // A booking departing yesterday occupies the previous-night slot in NewBook terms
+  // (arrival <= yesterday < departure=yesterday is strict, so it won't appear in prevBooking).
+  // We need it for the bracket and to prevent the vacant card spanning backward past a checkout.
+  const prevDepartedBooking = !prevBooking
+    ? departingBookingForSite(allBookings, siteId, yesterday)
+    : null
+
   // Determine flow type
   let flowType = 'vacant'
 
@@ -119,12 +126,18 @@ export function classifyRoom(site, allBookings, viewDate, yesterday, tomorrow, o
   // Previous-day bracket: use real NewBook status so the sliver remains 'arrived'
   // (wide, with checkout time) until the guest actually checks out and NewBook
   // flips the status to 'departed'. Do not override — let real status drive colour.
-  const prevStatus = prevBooking ? (prevBooking.booking_status || '').toLowerCase() : null
+  // prevStatus: prefer the actively-occupying booking; fall back to the one that departed yesterday
+  const prevStatus = prevBooking
+    ? (prevBooking.booking_status || '').toLowerCase()
+    : prevDepartedBooking
+      ? (prevDepartedBooking.booking_status || '').toLowerCase()
+      : null
 
   // Vacancy participates in the bracket system like bookings: a vacant room
   // spans into adjacent vacant days (no side border), while a day adjacent to
   // vacancy on an occupied room shows a grey 'vacant' bracket on that side.
-  const prevVacant = !prevBooking
+  // prevVacant: yesterday was truly empty — no occupant AND no departure that morning
+  const prevVacant = !prevBooking && !prevDepartedBooking
   const nextVacant = !nextBooking
   const isVacantFlow = !primaryBooking
 
@@ -150,7 +163,7 @@ export function classifyRoom(site, allBookings, viewDate, yesterday, tomorrow, o
       : !!primaryBooking && !!arrivalDate && arrivalDate < viewDate,
     spans_next: isVacantFlow ? nextVacant : spansNext,
 
-    previous_booking: prevBooking,
+    previous_booking: prevBooking ?? prevDepartedBooking,
     next_booking:     nextBooking,
     previous_status:  prevStatus ?? (prevVacant ? 'vacant' : null),
     next_status:      nextBooking ? (nextBooking.booking_status || '').toLowerCase() : 'vacant',