Mobile polish, trip detail preview, and nav memory Frontend: - Trip detail previews the newest expenses, with the full list a tap away - The Viajes tab returns to the trip you were reading, and pops back to the list once you are already in it - Confirmation banner after saves and deletes - Date range picker for trips + Spanish datepicker locale - Mobile spacing driven by tokens: containers own the vertical rhythm via gap, children carry no margins - Trip detail header clears the fold on a phone; trips list tightened Backend: - Birth date is now optional on registration and profile (migration: users.birth_date becomes nullable — runs on deploy) Tooling: - front-lint checks prettier too, matching what CI already ran