fix(mp): correct timezone handling on MP datetime writes

[chriskehayias]

Summary

• MP stores datetimes as wall-clock in the domain's time zone, not UTC. Existing write paths tagged values as UTC (${date}T00:00:00Z, new Date().toISOString()) which caused saved records to drift by the offset between the Node server's local zone and MP's domain zone — and edits compounded the drift on each save.
• Adds DomainTimezoneService (singleton, Windows→IANA mapping, toMpSqlDatetime / parseMpDatetime), a getMpTimezone() shared server action, fixes the offending writes in GroupService and FamilyService, and ships a reference doc + CLAUDE.md rule so future MP date work doesn't re-introduce the pattern.

Changes

• New: src/services/domainTimezoneService.ts — singleton boundary service; only path that reads MPHelper.getDomainInfo().TimeZoneName.
• New: src/services/domainTimezoneService.test.ts — 16 tests covering Windows→IANA mapping, caching, concurrent dedupe, wall-clock vs instant inputs, and a server-TZ-independent regression.
• New: src/components/shared-actions/domain.ts — getMpTimezone() server action for client-side Intl.DateTimeFormat rendering.
• Fix: src/services/groupService.ts — Start_Date / End_Date / Promotion_Date now route through toMpSqlDatetime.
• Fix: src/services/familyService.ts — Participant_Start_Date and donor Setup_Date writes use MP-TZ wall-clock instead of new Date().toISOString().
• Test: src/services/groupService.test.ts — mocks getDomainInfo, resets both singletons, asserts MP-SQL format, and adds a 3-save round-trip regression proving no drift.
• Docs: .claude/references/ministryplatform.datetimehandling.md — full reference (recipes, anti-patterns, Windows↔IANA table, testing guidance).
• Docs: CLAUDE.md — Key Development Practice chore(deps): security audit updates + mjml fix #12 + Reference Documents entry pointing to the new doc.

Test plan

• npx vitest run src/services/domainTimezoneService.test.ts — 16/16 pass
• npx vitest run (full suite) — 664/664 pass
• TZ=UTC npx vitest run — 664/664 pass
• TZ=America/Los_Angeles npx vitest run — 664/664 pass
• npm run lint — clean
• npx tsc --noEmit — no new errors (pre-existing errors in helper.test.ts, provider.test.ts, word-document.test.ts, logger.test.ts are untouched and unrelated)
• Manual spot-check: save a group with Start_Date = today, then re-save without changing fields, confirm date does not shift

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.50549% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/services/domainTimezoneService.ts	94.11%	5 Missing ⚠️

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