Import validates far enough that the fold cannot throw on imported data:
exactly one remainder-absorbing split per purchase, at least one split,
every split.postId resolving to a real post, every currency supported,
and every MonthId well-formed and in range (01-12). Over-budget and
percentages not summing to 100 remain permitted.
Tightens MONTH_ID to /^\d{4}-(0[1-9]|1[0-2])$/ (shape-only regex would
let "2026-13" through import and silently misfile inside the balance
fold, mirroring the defect already fixed in src/domain/months.ts).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
