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/components/list/DraftsList.tsx

@@ -1,12 +1,12 @@
 import { useT } from "../../i18n";
+import { formatClock, getDateLocale } from "../../lib/timeFormat";
 import type { DraftOut, RemoteDraftOut } from "../../lib/types";
 
 function timeLabel(iso: string): string {
   const d = new Date(iso);
   const now = new Date();
-  if (d.toDateString() === now.toDateString())
-    return d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" });
-  return d.toLocaleDateString([], { month: "short", day: "numeric" });
+  if (d.toDateString() === now.toDateString()) return formatClock(d);
+  return d.toLocaleDateString(getDateLocale(), { month: "short", day: "numeric" });
 }
 
 function DraftRow({

frontend/src/components/list/MessageList.tsx

@@ -2,17 +2,17 @@ import { useVirtualizer } from "@tanstack/react-virtual";
 import { useRef } from "react";
 
 import { useT } from "../../i18n";
+import { formatClock, getDateLocale } from "../../lib/timeFormat";
 import type { Address, ThreadSummary } from "../../lib/types";
 import { Icon } from "../Icon";
 
 function timeLabel(iso: string): string {
   const d = new Date(iso);
   const now = new Date();
-  if (d.toDateString() === now.toDateString())
-    return d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" });
+  if (d.toDateString() === now.toDateString()) return formatClock(d);
   if (d.getFullYear() === now.getFullYear())
-    return d.toLocaleDateString([], { month: "short", day: "numeric" });
-  return d.toLocaleDateString([], { year: "2-digit", month: "short", day: "numeric" });
+    return d.toLocaleDateString(getDateLocale(), { month: "short", day: "numeric" });
+  return d.toLocaleDateString(getDateLocale(), { year: "2-digit", month: "short", day: "numeric" });
 }
 
 // Stable per-sender hue so a sender's avatar keeps a consistent colour.