Add a reusable event calendar and put it on the front page

EventCalendar shows events on a month grid or as a list of the
month. Multi-day events are lane-packed bars that break at week
edges, series events appear on every meeting with their time, and
clicking a day lists everything on it. Phones get the list. Visitors
can narrow by scope, type, online only and search, all starting at
"all"; a page can pin section, host or type through props, which
hides that control.

Migration 019 rebuilds front_page_sections to allow a 'calendar'
band and slots it in after the retreats carousel, so it can be
reordered, retitled or hidden from the Front page editor.

useEvents drops its empty fallback so a failed request surfaces as
an error rather than an empty list, and returns the scope list
alongside the events for the calendar's scope filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/admin-schema.js

@@ -716,7 +716,7 @@ const frontPage = {
       columns: [
         enumeration(
           "section",
-          ["countdown", "retreats", "stats", "timeline", "connect"],
+          ["countdown", "retreats", "calendar", "stats", "timeline", "connect"],
           { required: true },
         ),
         text("title"),