fix(core): make day conversion deterministic

[dearlordylord]

Minor priority. This issue popped up when I was working on #19 (I'm not in London, as you can guess! 😄 ) (although there is A London, Ontario in my timezone!)

Summary

• Make getDay / convertToDay independent of the Node process timezone
• Preserve the behavior already implied by the existing tests: resolve the calendar day in Europe/London, then normalize it to noon UTC
• Add regression coverage for a case that fails when the process timezone is America/Los_Angeles

Notes

The previous implementation mixed local-time day reads with UTC writes, so the same timestamp could normalize to different days depending on where the tests ran. The existing tests already used Europe/London as the expected day, but they failed under non-London process timezones because the implementation depended on local Date behavior.

Test plan

• TZ=America/Los_Angeles node common/scripts/install-run-rush.js retest --to @hcengineering/core
• TZ=Europe/London node common/scripts/install-run-rush.js retest --to @hcengineering/core
• node_modules/.bin/eslint src/time.ts src/__tests__/time.test.ts
• node_modules/.bin/tsc --noEmit --pretty false