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>

components/landing/CopyLinkCard.tsx

@@ -0,0 +1,45 @@
+"use client";
+
+import { CheckIcon, LinkIcon } from "lucide-react";
+import { useState } from "react";
+import { cn } from "@/lib/utils";
+
+/**
+ * The one interactive subscribe option: copies the iCal URL for apps that
+ * want it pasted (Outlook and friends). Takes the shared card classes from the
+ * page so the three options stay identical.
+ */
+export function CopyLinkCard({ url, className }: { url: string; className: string }) {
+  const [state, setState] = useState<"idle" | "copied" | "failed">("idle");
+
+  const copy = async () => {
+    try {
+      await navigator.clipboard.writeText(url);
+      setState("copied");
+    } catch {
+      // No clipboard access (older browser, insecure context): show the URL instead of failing silently.
+      setState("failed");
+    }
+    setTimeout(() => setState("idle"), 2500);
+  };
+
+  return (
+    <button type="button" onClick={copy} className={className} aria-live="polite">
+      <span
+        className={cn(
+          "flex size-10 shrink-0 items-center justify-center rounded-lg transition-colors",
+          state === "copied" ? "bg-emerald-50 text-emerald-600" : "bg-primary/10 text-primary",
+        )}
+      >
+        {state === "copied" ? <CheckIcon aria-hidden className="size-5" /> : <LinkIcon aria-hidden className="size-5" />}
+      </span>
+      <span className="flex min-w-0 flex-col gap-0.5">
+        <span className="font-medium">{state === "copied" ? "Link kopiert" : "iCal-Link kopieren"}</span>
+        {/* break-all only for the URL (it has no spaces to wrap at); on words it splits mid-word. */}
+        <span className={cn("text-sm text-muted-foreground", state === "failed" && "break-all")}>
+          {state === "failed" ? url : "Outlook und andere Apps"}
+        </span>
+      </span>
+    </button>
+  );
+}