M4: Calendar plugin (builtin) + mail HTML/attachment fixes

Calendar plugin (webmail_plugin_calendar, non-removable core plugin):
- Local calendars/events CRUD with RLS isolation; recurrence expanded at
  query time (DST-correct), EXDATE + detached-override merge.
- Calendar app bundle: month/week/day/3-day views, mini-month, event form
  (color, show-as, timezone combobox, repeat, reminders, guests w/ recipient
  autocomplete, Jitsi video call), event popover with RSVP badges, resizable/
  collapsible left panel + theme toggle, overlap layout.
- iMIP invites: sdk.mail sends REQUEST from the organizer's mailbox (HTML +
  text/calendar + invite.ics); public /rsvp endpoint (vault-sealed token)
  records accepted/declined/tentative per attendee.
- "This and following" edit + scoped delete (this / this-and-following), with
  a delete confirmation dialog for recurring events.

Core:
- Settings: time_format (12/24h, default 24h) + jitsi_server; new Integrations
  section; date order follows time format (en-GB vs en-US) across Mail.
- Plugin SDK: mail (send from a user's account), vault (envelope encrypt),
  multi-method SSRF-guarded http; AppSwitcher product switcher; full-bleed
  PluginPage.

Mail HTML rendering:
- Sanitizer allows <style> blocks + class (CSS sanitized: drop script-like,
  strip @import/url() when remote images blocked); SANITIZER_VERSION 3 so
  already-indexed mail re-sanitizes on view.
- Attachments: a Content-Disposition: attachment part is a real attachment
  even when it also carries a Content-ID (fixes hidden invite.ics).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/plugins/test_calendar_recurrence.py

@@ -0,0 +1,84 @@
+"""Calendar recurrence expansion across a DST boundary (SPEC §A.3, acceptance A.7.2)."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta
+from zoneinfo import ZoneInfo
+
+from webmail_plugin_calendar.recurrence import expand_occurrences  # type: ignore[import-not-found]
+
+WARSAW = ZoneInfo("Europe/Warsaw")
+
+
+def _local(occ: list[tuple[datetime, datetime]]) -> dict[str, datetime]:
+    # Map "YYYY-MM-DD" (Warsaw date) -> the UTC start of that occurrence.
+    return {s.astimezone(WARSAW).date().isoformat(): s for s, _ in occ}
+
+
+def test_weekly_recurrence_keeps_local_time_across_dst() -> None:
+    # Monday 2026-03-23 10:00 Europe/Warsaw == 09:00 UTC (UTC+1, before DST).
+    # DST starts 2026-03-29, so from 03-30 the offset is UTC+2.
+    dtstart = datetime(2026, 3, 23, 9, 0, tzinfo=UTC)
+    occ = expand_occurrences(
+        dtstart=dtstart,
+        duration=timedelta(hours=1),
+        rrule="FREQ=WEEKLY;BYDAY=MO",
+        exdates=[],
+        range_start=datetime(2026, 3, 1, tzinfo=UTC),
+        range_end=datetime(2026, 4, 30, tzinfo=UTC),
+        tz="Europe/Warsaw",
+    )
+    by_date = _local(occ)
+
+    # Every occurrence is at 10:00 local wall-clock, on both sides of the DST change.
+    for start, _ in occ:
+        assert start.astimezone(WARSAW).strftime("%H:%M") == "10:00"
+
+    # ...but the UTC offset moves across the boundary.
+    assert by_date["2026-03-23"].hour == 9  # UTC+1
+    assert by_date["2026-03-30"].hour == 8  # UTC+2 (DST)
+    assert by_date["2026-04-06"].hour == 8
+
+
+def test_exdates_remove_instances() -> None:
+    dtstart = datetime(2026, 3, 23, 9, 0, tzinfo=UTC)
+    occ = expand_occurrences(
+        dtstart=dtstart,
+        duration=timedelta(hours=1),
+        rrule="FREQ=WEEKLY;BYDAY=MO",
+        # Exclude the 2026-03-30 occurrence (10:00 Warsaw == 08:00 UTC under DST).
+        exdates=[datetime(2026, 3, 30, 8, 0, tzinfo=UTC)],
+        range_start=datetime(2026, 3, 1, tzinfo=UTC),
+        range_end=datetime(2026, 4, 30, tzinfo=UTC),
+        tz="Europe/Warsaw",
+    )
+    dates = set(_local(occ))
+    assert "2026-03-23" in dates
+    assert "2026-03-30" not in dates  # excluded
+    assert "2026-04-06" in dates
+
+
+def test_single_event_no_rrule() -> None:
+    dtstart = datetime(2026, 6, 1, 12, 0, tzinfo=UTC)
+    occ = expand_occurrences(
+        dtstart=dtstart,
+        duration=timedelta(minutes=30),
+        rrule=None,
+        exdates=[],
+        range_start=datetime(2026, 6, 1, tzinfo=UTC),
+        range_end=datetime(2026, 6, 2, tzinfo=UTC),
+        tz="UTC",
+    )
+    assert occ == [(dtstart, dtstart + timedelta(minutes=30))]
+
+    # Outside the window -> nothing.
+    empty = expand_occurrences(
+        dtstart=dtstart,
+        duration=timedelta(minutes=30),
+        rrule=None,
+        exdates=[],
+        range_start=datetime(2026, 7, 1, tzinfo=UTC),
+        range_end=datetime(2026, 7, 2, tzinfo=UTC),
+        tz="UTC",
+    )
+    assert empty == []