fix(mobile): prevent bottom nav from clipping scrollable content and dialogs

- Add --bottom-nav-h CSS token (84px + safe-area on mobile, 0px on desktop)
  to give all fixes a single source of truth for the nav height
- Apply token to JourneySettingsDialog (fixes #650) and PlacesSidebar
  day-picker sheet so bottom-anchored sheets clear the nav bar
- Add paddingBottom to TripPlannerPage Bookings, Lists, and Budget tab
  scroll containers so content can be scrolled past the nav
- Bump Modal z-index from z-50 to z-[200] so modals render above the
  bottom nav (both share z-50 with nav winning by DOM order)

client/src/components/shared/Modal.tsx

@@ -50,7 +50,7 @@ export default function Modal({
 
   return (
     <div
-      className="fixed inset-0 z-50 flex items-start sm:items-center justify-center px-4 modal-backdrop"
+      className="fixed inset-0 z-[200] flex items-start sm:items-center justify-center px-4 modal-backdrop"
       style={{ backgroundColor: 'rgba(15, 23, 42, 0.5)', paddingTop: 70, paddingBottom: 20, overflow: 'hidden' }}
       onMouseDown={e => { mouseDownTarget.current = e.target }}
       onClick={e => {