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/app/plugins/vault.py

@@ -0,0 +1,21 @@
+"""Secret encryption exposed to plugins (SPEC §10.2 vault).
+
+Heavy plugins (calendar/contacts) store third-party credentials — CalDAV/CardDAV
+passwords, ICS tokens — in their own ``secret_ct bytea`` columns. They encrypt via
+this facade so those secrets are protected at rest with the same envelope scheme as
+core account credentials, without importing ``app.security`` directly.
+"""
+
+from __future__ import annotations
+
+from app.security.crypto import decrypt_secret, encrypt_secret
+
+
+class PluginVault:
+    def encrypt(self, plaintext: bytes) -> bytes:
+        """Envelope-encrypt ``plaintext`` for storage in a ``secret_ct`` column."""
+        return encrypt_secret(plaintext)
+
+    def decrypt(self, ciphertext: bytes) -> bytes:
+        """Reverse :meth:`encrypt`. Raises on a wrong/rotated master key."""
+        return decrypt_secret(ciphertext)