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/tests/indexer/test_mime.py

@@ -126,6 +126,32 @@ def test_inline_image_nested_in_related_is_extracted() -> None:
     assert parsed.has_attachments is False
 
 
+def test_attachment_with_content_id_is_a_real_attachment() -> None:
+    # iMIP invites (e.g. Fastmail) send invite.ics as Content-Disposition: attachment
+    # yet also give it a Content-ID. It must show as a downloadable chip, not be
+    # hidden as an inline resource (regression: "no attachments" shown).
+    from email.mime.multipart import MIMEMultipart
+    from email.mime.text import MIMEText
+
+    root = MIMEMultipart("mixed")
+    root["Subject"] = "invite"
+    root["From"] = "org@example.com"
+    root["To"] = "me@example.com"
+    root["Message-ID"] = "<inv@x.com>"
+    root.attach(MIMEText("body", "plain"))
+    ics = MIMEText("BEGIN:VCALENDAR\nEND:VCALENDAR", "calendar")
+    ics.add_header("Content-Disposition", "attachment", filename="invite.ics")
+    ics.add_header("Content-ID", "<cal123@x>")
+    root.attach(ics)
+
+    parsed = parse_message(root.as_bytes())
+    assert len(parsed.attachments) == 1
+    att = parsed.attachments[0]
+    assert att.filename == "invite.ics"
+    assert att.content_id is None  # not treated as inline despite the Content-ID
+    assert parsed.has_attachments is True
+
+
 def test_missing_message_id_returns_none() -> None:
     msg = EmailMessage()
     msg["Subject"] = "no id"