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>

frontend/src/i18n/en.ts

@@ -223,6 +223,13 @@ export const en = {
   "settings.revoke": "Revoke",
   "settings.threadOrder": "Conversation order",
   "settings.timezone": "Time zone",
+  "settings.timeFormat": "Time format",
+  "settings.timeFormat24": "24-hour (14:30)",
+  "settings.timeFormat12": "12-hour (2:30 PM)",
+  "settings.integrations": "Integrations",
+  "settings.jitsi": "Jitsi Meet",
+  "settings.jitsiHint":
+    "Server used to generate video-call links for calendar events. Leave empty to use the public https://meet.jit.si/ server.",
   "settings.addAccount": "Add account",
   "settings.syncState": "State: {state}",
   "settings.confirmRemoveAccount": "Remove this account and all its mail?",