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/sdk.py

@@ -29,7 +29,9 @@ from app.models import User
 from app.outbox import service as outbox_service
 from app.plugins.actions import PluginActions, PluginMessages
 from app.plugins.db import plugin_session
+from app.plugins.http import PluginHttp
 from app.plugins.kv import PluginKvStore
+from app.plugins.mail import PluginMail
 from app.plugins.types import (
     Hook,
     JobFunc,
@@ -40,6 +42,7 @@ from app.plugins.types import (
     Slot,
     UiManifest,
 )
+from app.plugins.vault import PluginVault
 from app.queue import enqueue_job
 
 __all__ = [
@@ -122,6 +125,11 @@ class PluginSdk:
         return outbox_service
 
     # --- messages -----------------------------------------------------------
+    @property
+    def mail(self) -> PluginMail:
+        """Send an email from one of the user's own accounts (iMIP invites, …)."""
+        return PluginMail()
+
     @property
     def messages(self) -> PluginMessages:
         """Read a user's message envelope (subject/sender/flags) for matching."""
@@ -137,6 +145,19 @@ class PluginSdk:
         """GET ``url`` through the §10.6 SSRF guard (blocks private IPs/oversize)."""
         return await guarded_get(url)
 
+    @property
+    def http(self) -> PluginHttp:
+        """Multi-method SSRF-guarded HTTP (GET/PUT/DELETE/PROPFIND/REPORT) for
+        CalDAV/CardDAV/ICS. Returns status + headers + body."""
+        return PluginHttp()
+
+    # --- secret vault -------------------------------------------------------
+    @property
+    def vault(self) -> PluginVault:
+        """Envelope encrypt/decrypt for a plugin's own ``secret_ct`` columns
+        (CalDAV/CardDAV credentials, tokens) — §10.2 at-rest protection."""
+        return PluginVault()
+
 
 def sdk(name: str) -> PluginSdk:
     """Return the SDK facade bound to plugin ``name`` (SPEC §11)."""