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/events.test.ts

@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+import { parseApiEvent, parseApiEvents } from "@/lib/events";
+import type { ApiEvent } from "@/lib/types";
+
+const row = (overrides: Partial<ApiEvent> = {}): ApiEvent => ({
+  eventId: 7,
+  calendarId: 5,
+  uuid: "u-7",
+  name: "Anna",
+  description: null,
+  startDateTime: "1990-10-02T22:00:00.000Z",
+  endDateTime: "1990-10-03T21:59:00.000Z",
+  createdDate: "2024-01-01T10:00:00.000Z",
+  lastModifiedDate: "2024-02-01T10:00:00.000Z",
+  location: null,
+  createdBy: null,
+  lastModifiedBy: "Paula",
+  url: null,
+  wholeDay: 1,
+  repeatFrequency: "YEARLY",
+  status: "DRAFT",
+  ...overrides,
+});
+
+describe("parseApiEvent", () => {
+  it("coerces the raw row: 1 → true, nulls → empty strings, ISO → Date", () => {
+    const event = parseApiEvent(row());
+    expect(event).toMatchObject({
+      eventId: 7,
+      wholeDay: true,
+      description: "",
+      location: "",
+      url: "",
+      createdBy: "",
+      lastModifiedBy: "Paula",
+      repeatFrequency: "YEARLY",
+      status: "DRAFT",
+    });
+    expect(event?.start.toISOString()).toBe("1990-10-02T22:00:00.000Z");
+  });
+
+  it("drops deleted events", () => {
+    expect(parseApiEvent(row({ status: "DELETED" }))).toBeNull();
+  });
+
+  it("drops a row without a usable start or end - what a deleted version looks like", () => {
+    expect(parseApiEvent(row({ status: null, startDateTime: null }))).toBeNull();
+    expect(parseApiEvent(row({ endDateTime: "not a date" }))).toBeNull();
+  });
+
+  it("reads a missing status as PUBLIC (the public listing omits it)", () => {
+    const withoutStatus = row();
+    delete withoutStatus.status;
+    expect(parseApiEvent(withoutStatus)?.status).toBe("PUBLIC");
+  });
+
+  it("never promotes an unknown status to PUBLIC", () => {
+    expect(parseApiEvent(row({ status: "ARCHIVED" }))?.status).toBe("PRIVATE");
+  });
+
+  it("reads 0 and null as not whole-day", () => {
+    expect(parseApiEvent(row({ wholeDay: 0 }))?.wholeDay).toBe(false);
+    expect(parseApiEvent(row({ wholeDay: null }))?.wholeDay).toBe(false);
+  });
+});
+
+describe("parseApiEvents", () => {
+  it("keeps only the rows worth showing", () => {
+    const events = parseApiEvents([row({ eventId: 1 }), row({ eventId: 2, status: "DELETED" }), row({ eventId: 3 })]);
+    expect(events.map((e) => e.eventId)).toEqual([1, 3]);
+  });
+});