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/api/settings.py

@@ -18,17 +18,22 @@ router = APIRouter(prefix="/settings", tags=["settings"])
 DbSession = Annotated[AsyncSession, Depends(get_session)]
 
 _ALLOWED_ORDER = {"asc", "desc"}
+_ALLOWED_TIME_FORMAT = {"12", "24"}
 
 
 class SettingsOut(BaseModel):
     thread_order: str = "asc"
     tz: str = "UTC"
+    time_format: str = "24"  # 12 = AM/PM, 24 = 24-hour (default); applies app-wide
+    jitsi_server: str = ""  # blank => https://meet.jit.si/
     allowed_image_senders: list[str] = []
 
 
 class SettingsPatch(BaseModel):
     thread_order: str | None = None
     tz: str | None = None
+    time_format: str | None = None
+    jitsi_server: str | None = None
 
 
 class QuotaOut(BaseModel):
@@ -56,6 +61,8 @@ def _settings_out(user: User) -> SettingsOut:
     return SettingsOut(
         thread_order=str(s.get("thread_order", "asc")),
         tz=str(s.get("tz", "UTC")),
+        time_format=str(s.get("time_format", "24")),
+        jitsi_server=str(s.get("jitsi_server", "")),
         allowed_image_senders=sorted(allowed_image_senders(user)),
     )
 
@@ -81,6 +88,10 @@ async def patch_settings(body: SettingsPatch, user: CurrentUser, db: DbSession)
         updates["thread_order"] = body.thread_order
     if body.tz is not None:
         updates["tz"] = body.tz
+    if body.time_format is not None and body.time_format in _ALLOWED_TIME_FORMAT:
+        updates["time_format"] = body.time_format
+    if body.jitsi_server is not None:
+        updates["jitsi_server"] = body.jitsi_server.strip()
     user.settings = updates
     await db.commit()
     return _settings_out(user)