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>

src/lib/eventSeries.ts

@@ -171,6 +171,39 @@ export function upcomingOccurrences(
   return out
 }
 
+/** Every meeting between two dates, inclusive, in order. For a
+ *  calendar page: `from` and `to` are the visible range. */
+export function occurrencesBetween(
+  series: EventSeries | null | undefined,
+  startsOn: string | null | undefined,
+  endsOn: string | null | undefined,
+  from: string,
+  to: string,
+): string[] {
+  if (!series) return []
+  const out: string[] = []
+  for (const date of occurrences(series, startsOn, endsOn)) {
+    if (date > to) break
+    if (date >= from) out.push(date)
+  }
+  return out
+}
+
+/** The first meeting on or after `from`, or null if the series has
+ *  ended by then. */
+export function firstOccurrenceFrom(
+  series: EventSeries | null | undefined,
+  startsOn: string | null | undefined,
+  endsOn: string | null | undefined,
+  from: string,
+): string | null {
+  if (!series) return null
+  for (const date of occurrences(series, startsOn, endsOn)) {
+    if (date >= from) return date
+  }
+  return null
+}
+
 /* ── Words ───────────────────────────────────────────────────── */
 
 const ORDINALS = ['', '1st', '2nd', '3rd', '4th', 'last']