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/ui/sonner.tsx

@@ -0,0 +1,48 @@
+"use client"
+
+import { Toaster as Sonner, type ToasterProps } from "sonner"
+import { CircleCheckIcon, InfoIcon, TriangleAlertIcon, OctagonXIcon, Loader2Icon } from "lucide-react"
+
+// Light only, like the rest of the app (globals.css) - the generated version
+// followed the OS theme via next-themes, which gave dark toasts on a light page.
+const Toaster = ({ ...props }: ToasterProps) => {
+  return (
+    <Sonner
+      theme="light"
+      className="toaster group"
+      icons={{
+        success: (
+          <CircleCheckIcon className="size-4" />
+        ),
+        info: (
+          <InfoIcon className="size-4" />
+        ),
+        warning: (
+          <TriangleAlertIcon className="size-4" />
+        ),
+        error: (
+          <OctagonXIcon className="size-4" />
+        ),
+        loading: (
+          <Loader2Icon className="size-4 animate-spin" />
+        ),
+      }}
+      style={
+        {
+          "--normal-bg": "var(--popover)",
+          "--normal-text": "var(--popover-foreground)",
+          "--normal-border": "var(--border)",
+          "--border-radius": "var(--radius)",
+        } as React.CSSProperties
+      }
+      toastOptions={{
+        classNames: {
+          toast: "cn-toast",
+        },
+      }}
+      {...props}
+    />
+  )
+}
+
+export { Toaster }