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/filter.test.ts

@@ -0,0 +1,107 @@
+import { describe, expect, it } from "vitest";
+import { fromWallTime } from "@/lib/dates";
+import { filterEvents, groupByMonth, matchesQuery, matchesRange, sortEvents } from "@/lib/filter";
+import { makeEvent } from "@/lib/test-fixtures";
+
+const now = new Date("2026-09-27T10:00:00Z"); // 12:00 Berlin
+
+describe("matchesRange", () => {
+  it("keeps an event that ended earlier today in 'future' until midnight", () => {
+    const earlierToday = makeEvent({
+      start: fromWallTime("2026-09-27", "08:00"),
+      end: fromWallTime("2026-09-27", "09:00"),
+    });
+    expect(matchesRange(earlierToday, "future", now)).toBe(true);
+    expect(matchesRange(earlierToday, "past", now)).toBe(false);
+  });
+
+  it("puts yesterday's event in 'past'", () => {
+    const yesterday = makeEvent({ start: fromWallTime("2026-09-26", "19:00"), end: fromWallTime("2026-09-26", "21:00") });
+    expect(matchesRange(yesterday, "past", now)).toBe(true);
+    expect(matchesRange(yesterday, "future", now)).toBe(false);
+  });
+
+  it("treats any repeating event as current, whatever its calendar", () => {
+    const old = { start: fromWallTime("1990-01-01", "00:00"), end: fromWallTime("1990-01-01", "23:59") };
+    const birthday = makeEvent({ ...old, calendarId: 5, repeatFrequency: "YEARLY" });
+    const weeklyRehearsal = makeEvent({ ...old, calendarId: 4, repeatFrequency: "WEEKLY" });
+    for (const e of [birthday, weeklyRehearsal]) {
+      expect(matchesRange(e, "future", now)).toBe(true);
+      expect(matchesRange(e, "past", now)).toBe(false);
+      expect(matchesRange(e, "all", now)).toBe(true);
+    }
+  });
+
+  it("no longer exempts a non-repeating event just for being in Birthdays", () => {
+    const oneOff = makeEvent({ calendarId: 5, start: fromWallTime("2020-01-01", "00:00"), end: fromWallTime("2020-01-01", "23:59") });
+    expect(matchesRange(oneOff, "future", now)).toBe(false);
+  });
+});
+
+describe("matchesQuery", () => {
+  const event = makeEvent({ name: "Probe Weihnachtskonzert", location: "Gemeindehaus Süd" });
+
+  it("matches title and location, case-insensitively, all words", () => {
+    expect(matchesQuery(event, "")).toBe(true);
+    expect(matchesQuery(event, "weihnacht")).toBe(true);
+    expect(matchesQuery(event, "PROBE süd")).toBe(true);
+    expect(matchesQuery(event, "probe nord")).toBe(false);
+  });
+});
+
+describe("filterEvents", () => {
+  it("combines calendar, range and search", () => {
+    const events = [
+      makeEvent({ eventId: 1, calendarId: 1, name: "Konzert" }),
+      makeEvent({ eventId: 2, calendarId: 2, name: "Konzert" }),
+      makeEvent({ eventId: 3, calendarId: 1, name: "Probe" }),
+    ];
+    const result = filterEvents(events, { calendarIds: [1], range: "future", query: "konz" }, now);
+    expect(result.map((e) => e.eventId)).toEqual([1]);
+  });
+});
+
+describe("sortEvents", () => {
+  const a = makeEvent({ eventId: 1, name: "B", start: fromWallTime("2026-10-02", "19:00") });
+  const b = makeEvent({ eventId: 2, name: "A", start: fromWallTime("2026-10-02", "19:00") });
+  const c = makeEvent({ eventId: 3, name: "C", start: fromWallTime("2026-10-01", "19:00") });
+
+  it("orders by start and breaks ties by title", () => {
+    expect(sortEvents([a, b, c], "start_asc", now).map((e) => e.eventId)).toEqual([3, 2, 1]);
+    // Descending flips the dates only; equal starts still read A before B.
+    expect(sortEvents([a, b, c], "start_desc", now).map((e) => e.eventId)).toEqual([2, 1, 3]);
+  });
+
+  it("does not mutate its input", () => {
+    const input = [a, b, c];
+    sortEvents(input, "start_asc", now);
+    expect(input.map((e) => e.eventId)).toEqual([1, 2, 3]);
+  });
+
+  it("sorts a birthday by its next occurrence, not its year", () => {
+    const birthday = makeEvent({ eventId: 9, repeatFrequency: "YEARLY", start: fromWallTime("1990-10-01", "12:00"), end: fromWallTime("1990-10-01", "23:59") });
+    const concert = makeEvent({ eventId: 8, start: fromWallTime("2026-10-02", "19:00") });
+    expect(sortEvents([concert, birthday], "start_asc", now).map((e) => e.eventId)).toEqual([9, 8]);
+  });
+
+  it("sorts by created date, missing ones first", () => {
+    const x = makeEvent({ eventId: 1, created: new Date("2026-05-01T00:00:00Z") });
+    const y = makeEvent({ eventId: 2, created: null });
+    expect(sortEvents([x, y], "created_asc", now).map((e) => e.eventId)).toEqual([2, 1]);
+  });
+});
+
+describe("groupByMonth", () => {
+  it("groups consecutive events by Berlin month, labelled in German", () => {
+    const events = [
+      makeEvent({ eventId: 1, start: fromWallTime("2026-10-01", "00:30") }), // still 30 Sep in UTC
+      makeEvent({ eventId: 2, start: fromWallTime("2026-10-20", "19:00") }),
+      makeEvent({ eventId: 3, start: fromWallTime("2026-11-02", "19:00") }),
+    ];
+    const groups = groupByMonth(events, now);
+    expect(groups.map((g) => [g.key, g.label, g.events.map((e) => e.eventId)])).toEqual([
+      ["2026-10", "Oktober 2026", [1, 2]],
+      ["2026-11", "November 2026", [3]],
+    ]);
+  });
+});