Rewrite the calendar frontend in Next.js

Replaces the Angular 18 app with Next.js 16 / React 19 / Tailwind 4 / shadcn,
the same stack and design system as nachklang-feedback, planned in
docs/rewrite-plan.md.

- Admin: all calendars in one mobile-friendly list (month groups, calendar
  chips, Kommend/Vergangen/Alle, search, sort, filters in the URL), an edit
  sheet for create/edit/duplicate/delete, publish confirmation, and an
  "Abonnieren" dialog with subscription links for every calendar.
- Public landing page: subscribe links from env instead of hardcoded, plus
  the next public events, server-rendered and revalidated every 5 minutes.
- All times handled explicitly as Europe/Berlin; whole-day events stored as
  00:00-23:59. Fixes carried over from the Angular app: the move dialog
  leaking a cancelled calendar change, saves that looked saved when they
  failed, in-place Date mutation, and duplicated calendar-id mappings.
- Same shared-session auth as before (proxy.ts gate, /admin/me, 401 vs 403).
- 112 Vitest tests over lib/; docs in docs/ (architecture, features,
  gotchas, API wish list, deployment).

Needs the API branch feature/calendar-ical-escaping-subscriptions deployed
first (GET /calendar/events/subscriptions).

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

lib/test-fixtures.ts

@@ -0,0 +1,23 @@
+import { fromWallTime } from "@/lib/dates";
+import type { CalendarEvent } from "@/lib/types";
+
+/** A plain, published, timed event; override what the test is about. Test-only. */
+export const makeEvent = (overrides: Partial<CalendarEvent> = {}): CalendarEvent => ({
+  eventId: 1,
+  calendarId: 1,
+  uuid: "uuid-1",
+  name: "Konzert",
+  description: "",
+  start: fromWallTime("2026-10-03", "19:00"),
+  end: fromWallTime("2026-10-03", "21:00"),
+  created: new Date("2026-01-01T12:00:00Z"),
+  lastModified: null,
+  location: "Karlsruhe",
+  createdBy: "",
+  lastModifiedBy: "",
+  url: "",
+  wholeDay: false,
+  repeatFrequency: "",
+  status: "PUBLIC",
+  ...overrides,
+});