fix(mail, calendar): speak UTC on the wire, render in the viewer's time zone

[s-aga-r]

Closes #56

Problem

Timestamps flowed through three different conventions: Stalwart speaks UTC (...Z or offset forms like -05:00), the virtual doctypes converted that into naive system-timezone strings, and the mail/calendar UIs then parsed those strings in the browser's zone. Whenever browser ≠ system zone, every displayed time was wrong — and calendar events pinned themselves to their stored wall clock (a 2–4 PM Asia/Kolkata event still showed 2–4 PM from Vienna).

Approach

One contract everywhere:

• APIs speak and listen UTC ...Z (second precision). normalize_utc_z converts Stalwart's offset form and reads naive wire values as UTC. Virtual doctypes store/serve UTC; filters (before/after) listen UTC.
• MariaDB-backed Datetime fields keep system time — to_utc_z/from_utc_z convert at the wire boundary.
• UIs convert for display: browser zone → User.time_zone → system zone (get_user_info also exposes system_time_zone so plain DB fields render correctly).
• JSCalendar stays local + IANA zone per RFC 8984; the UI converts events into the viewer's zone via fromEventZone, and edits/drags re-zone the event to the viewer at the same instant (recurring-master passthroughs keep the master's zone).

Highlights

• Mail messages, push subscriptions, vacation responses, contact cards, calendar events/notifications and the admin API's raw Stalwart pass-throughs are all normalized; the inbound sync API loses its system-tz round trip.
• Calendar alert AbsoluteTrigger.when round-trips as UTC (was written naive-local, read as UTC — alerts drifted by the viewer's offset). Recurrence until is a spec-correct local datetime (the forced Z shifted the picked date a day east of UTC).
• ICS exports: VTIMEZONE components are now emitted for every referenced TZID (Outlook read our invites in its own zone), all-day series emit DATE-valued UNTIL, absolute alarm triggers are UTC, and instance CANCELs carry the occurrence's DTSTART.
• Search/export date filters resolve day bounds in the user's zone instead of UTC midnight (IST users silently lost the first 5½ hours of a searched day).
• parse_iso_datetime reads naive values as UTC instead of the OS process zone; RSVP expiry no longer depends on the container's zone; server-rendered text (forward/quoted headers, MIME dialog) converts via to_user_timezone.

Tests

• New suite/mail/tests/test_dt.py (16 site-free unit tests over the four dt helpers — pass identically under TZ=America/New_York).
• New frontend/src/apps/calendar/utils/datetime.test.ts + extended mail datetime tests; full frontend suite: 1,024 tests green.
• ICS output verified against icalendar 6.1.3 (VTIMEZONE presence, UTC/DATE UNTIL, instance DTSTART).