feat: collapsible day detail panel in planner

Adds a collapse/expand toggle to the day detail panel header.
Collapsed state persists across day switches. Clicking the header
or the chevron button toggles between compact header-only view
and the full detail panel.

Closes #457

client/src/pages/TripPlannerPage.tsx

@@ -154,6 +154,7 @@ export default function TripPlannerPage(): React.ReactElement | null {
   const { leftWidth, rightWidth, leftCollapsed, rightCollapsed, setLeftCollapsed, setRightCollapsed, startResizeLeft, startResizeRight } = useResizablePanels()
   const { selectedPlaceId, selectedAssignmentId, setSelectedPlaceId, selectAssignment } = usePlaceSelection()
   const [showDayDetail, setShowDayDetail] = useState<Day | null>(null)
+  const [dayDetailCollapsed, setDayDetailCollapsed] = useState(false)
   const [showPlaceForm, setShowPlaceForm] = useState<boolean>(false)
   const [editingPlace, setEditingPlace] = useState<Place | null>(null)
   const [prefillCoords, setPrefillCoords] = useState<{ lat: number; lng: number; name?: string; address?: string } | null>(null)
@@ -766,6 +767,8 @@ export default function TripPlannerPage(): React.ReactElement | null {
                   onAccommodationChange={loadAccommodations}
                   leftWidth={isMobile ? 0 : (leftCollapsed ? 0 : leftWidth)}
                   rightWidth={isMobile ? 0 : (rightCollapsed ? 0 : rightWidth)}
+                  collapsed={dayDetailCollapsed}
+                  onToggleCollapse={() => setDayDetailCollapsed(c => !c)}
                 />
               )
             })()}