fix: keep the calendar day view working when midnight is skipped by DST

[Mrdifferent2022]

Problem

The calendar events query built its day range with zonedInstant(date, "00:00", zone), which deliberately throws for nonexistent local times. On days when a zone springs forward at midnight (e.g. America/Santiago goes 00:00→01:00 on 2026-09-06), the promise rejected, the effect showed the "This time does not exist" error, and the user saw zero events for that day (in 30-day mode the whole query failed).

A day boundary is a range-computation detail, not a user-entered appointment time, so rejecting it serves no purpose.

Fix

Add startOfZonedDay in date-time.ts: resolve 00:00, and when a DST gap removes midnight, clamp to the first existing local time that day. The calendar query uses it for both range ends; event creation still uses zonedInstant and keeps rejecting gap times.

Verification

• New tests in apps/mobile/test/date-time.test.ts: the Santiago spring-forward day clamps to 01:00 local (04:00Z), ordinary days still start at midnight.
• pnpm --dir apps/mobile typecheck passes and the mobile unit tests pass.
• Untested path (stated per CONTRIBUTING): rendered CalendarScreen behavior on a DST day; verified at the startOfZonedDay level instead.

Co-authored-by: Claude noreply@anthropic.com

[jerelvelarde]

Value: day-range queries work when DST skips midnight, while appointment entry still rejects nonexistent times. Six date-time tests passed. Template: verification and rendered-screen limitation stated; UI evidence would complete the description. Updated-main diff reviewed; no actionable security finding. Merge after required CI.

[jerelvelarde]

Re-reviewed the formatting-only revision. Six date-time tests and Biome on all changed files pass. Previous value/template/security assessment holds. Merge after all required CI checks pass.