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>

plugins/webmail_plugin_calendar/pyproject.toml

@@ -0,0 +1,24 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "webmail-plugin-calendar"
+version = "0.1.0"
+description = "Calendar — the reference heavy plugin (SPEC Appendix A). Local events, iMIP, CalDAV/ICS."
+requires-python = ">=3.11"
+# Plugin owns its dependencies (SPEC §A.2). caldav is only needed for A3.
+dependencies = [
+  "python-dateutil>=2.9",
+  "icalendar>=5.0,<7",
+  "tzdata",
+]
+
+[project.entry-points."webmail.plugins"]
+calendar = "webmail_plugin_calendar:plugin"
+
+[tool.setuptools]
+packages = ["webmail_plugin_calendar"]
+
+[tool.setuptools.package-data]
+webmail_plugin_calendar = ["ui/*.js", "migrations/versions/*.py"]