[1141] Add locale-aware display date adapter - #2743

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Fresh-context review record (coordinator, 2026-09-06; read-only Opus 5 reviewer over the diff against the merge base; route: the standing D-12 rule for Codex-session PRs — review, fix MEDIUM+ in-PR, merge, disposition comment).

VERDICT: SHIP, nothing fixed in-PR. The adapter is new and unwired (no shipped surface changes), so the findings are contract traps for the future migration and are recorded on #1141 rather than fixed here.

Tracked for #1141: MEDIUM-1 — the guard routes only the naked YYYY-MM-DD form to the UTC-safe path, while the persisted due-date shape per dueDates.ts is an ISO DateTimeOffset, which is accepted and formatted in the browser zone (a UTC-5 user sees "Feb 28" for a Feb 29 due date, the exact shift dueDates.ts exists to prevent); MEDIUM-2 — formatDisplayCalendarDate returns null for that persisted form, so a like-for-like migration of CardItem.vue, PaperBoardCard.vue, useTodayDossier.ts blanks every due-date badge (pinned as intended by the spec, but undocumented); MEDIUM-3 (unverified) — dateStyle spread over year/month/day defaults throws in ECMA-402 and the catch swallows it to null. LOWs: no region resolution (the ADR-0054 en-GB regression); the spec computes expected with the same Intl call as the code (tautological) and assumes full-ICU Node.