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/thread/ThreadView.tsx

@@ -4,6 +4,7 @@ import { Fragment, useEffect, useMemo, useRef, useState } from "react";
 import { useT } from "../../i18n";
 import { api } from "../../lib/api";
 import { printMessages } from "../../lib/print";
+import { getDateLocale, getHour12 } from "../../lib/timeFormat";
 import type { AccountOut, MailboxOut, MessageEnvelope } from "../../lib/types";
 import { PluginSlot } from "../../plugins/PluginSlot";
 import { Composer, type ComposerInit } from "../compose/Composer";
@@ -39,13 +40,17 @@ function recipientSummary(addrs: { name: string; email: string }[]): string {
 
 function attributionFor(msg: MessageEnvelope): string {
   const d = new Date(msg.date);
-  const date = d.toLocaleDateString("en-US", {
+  const date = d.toLocaleDateString(getDateLocale(), {
     weekday: "short",
     month: "short",
     day: "numeric",
     year: "numeric",
   });
-  const time = d.toLocaleTimeString("en-US", { hour: "numeric", minute: "2-digit" });
+  const time = d.toLocaleTimeString("en-US", {
+    hour: "numeric",
+    minute: "2-digit",
+    hour12: getHour12(),
+  });
   const who = msg.from_addr[0]?.name || msg.from_addr[0]?.email || "";
   return `On ${date}, at ${time}, ${who} wrote:`;
 }
@@ -110,13 +115,14 @@ function fmtRelative(iso: string): string {
 }
 
 function fmtFull(iso: string): string {
-  return new Date(iso).toLocaleString("en-US", {
+  return new Date(iso).toLocaleString(getDateLocale(), {
     weekday: "short",
     year: "numeric",
     month: "short",
     day: "numeric",
     hour: "numeric",
     minute: "2-digit",
+    hour12: getHour12(),
   });
 }