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"),

server/src/migrations/019_front_page_calendar.sql

@@ -0,0 +1,61 @@
+-- ═══════════════════════════════════════════════════════════════
+-- FRONT PAGE: calendar band
+--
+-- Adds 'calendar' to the sections the front page can draw. The key
+-- is a CHECK, and SQLite can't alter a CHECK in place, so the table
+-- is rebuilt: new table, copy, drop, rename.
+--
+-- No PRAGMA foreign_keys dance. front_page_sections only points out
+-- (at front_page); nothing points in, so dropping the old table
+-- cascades into nothing, and the copy keeps every page_id valid.
+--
+-- The new band is inserted straight after the retreats carousel,
+-- where "what's on" reads naturally, by shifting everything below it
+-- down one. If retreats was removed on this box, it goes last.
+--
+-- Adding another section later is the same three steps: this CHECK,
+-- the enum in both descriptor halves, and SECTIONS in Home.tsx.
+--
+-- No BEGIN...END in this file.
+-- ═══════════════════════════════════════════════════════════════
+
+CREATE TABLE front_page_sections_new (
+  id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  page_id     TEXT NOT NULL REFERENCES front_page (id) ON DELETE CASCADE,
+  sort_order  INTEGER NOT NULL DEFAULT 0,
+  section     TEXT NOT NULL
+                CHECK (section IN ('countdown', 'retreats', 'calendar', 'stats',
+                                   'timeline', 'connect')),
+  title       TEXT,
+  blurb       TEXT,
+  is_hidden   INTEGER NOT NULL DEFAULT 0 CHECK (is_hidden IN (0, 1)),
+  UNIQUE (page_id, section)
+) STRICT;
+
+INSERT INTO front_page_sections_new (id, page_id, sort_order, section, title, blurb, is_hidden)
+SELECT id, page_id, sort_order, section, title, blurb, is_hidden
+  FROM front_page_sections;
+
+DROP TABLE front_page_sections;
+
+ALTER TABLE front_page_sections_new RENAME TO front_page_sections;
+
+UPDATE front_page_sections
+   SET sort_order = sort_order + 1
+ WHERE page_id = 'home'
+   AND sort_order > COALESCE(
+         (SELECT sort_order FROM front_page_sections
+           WHERE page_id = 'home' AND section = 'retreats'),
+         (SELECT MAX(sort_order) FROM front_page_sections WHERE page_id = 'home'));
+
+INSERT INTO front_page_sections (page_id, sort_order, section, title, blurb)
+SELECT 'home',
+       COALESCE(
+         (SELECT sort_order + 1 FROM front_page_sections
+           WHERE page_id = 'home' AND section = 'retreats'),
+         (SELECT COALESCE(MAX(sort_order), -1) + 1 FROM front_page_sections
+           WHERE page_id = 'home')),
+       'calendar',
+       'What''s on',
+       'Every gathering, class and meeting in one place.'
+ WHERE EXISTS (SELECT 1 FROM front_page WHERE id = 'home');