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/dialog.tsx

@@ -0,0 +1,160 @@
+"use client"
+
+import * as React from "react"
+import { Dialog as DialogPrimitive } from "@base-ui/react/dialog"
+import { cn } from "@/lib/utils"
+
+import { Button } from "@/components/ui/button"
+import { XIcon } from "lucide-react"
+
+function Dialog({ ...props }: DialogPrimitive.Root.Props) {
+  return <DialogPrimitive.Root data-slot="dialog" {...props} />
+}
+
+function DialogTrigger({ ...props }: DialogPrimitive.Trigger.Props) {
+  return <DialogPrimitive.Trigger data-slot="dialog-trigger" {...props} />
+}
+
+function DialogPortal({ ...props }: DialogPrimitive.Portal.Props) {
+  return <DialogPrimitive.Portal data-slot="dialog-portal" {...props} />
+}
+
+function DialogClose({ ...props }: DialogPrimitive.Close.Props) {
+  return <DialogPrimitive.Close data-slot="dialog-close" {...props} />
+}
+
+function DialogOverlay({
+  className,
+  ...props
+}: DialogPrimitive.Backdrop.Props) {
+  return (
+    <DialogPrimitive.Backdrop
+      data-slot="dialog-overlay"
+      className={cn(
+        "fixed inset-0 isolate z-50 bg-black/30 duration-100 supports-backdrop-filter:backdrop-blur-sm data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function DialogContent({
+  className,
+  children,
+  showCloseButton = true,
+  ...props
+}: DialogPrimitive.Popup.Props & {
+  showCloseButton?: boolean
+}) {
+  return (
+    <DialogPortal>
+      <DialogOverlay />
+      <DialogPrimitive.Popup
+        data-slot="dialog-content"
+        className={cn(
+          "fixed top-1/2 left-1/2 z-50 grid w-full max-w-[calc(100%-2rem)] -translate-x-1/2 -translate-y-1/2 gap-6 rounded-4xl bg-popover p-6 text-sm text-popover-foreground shadow-xl ring-1 ring-foreground/5 duration-100 outline-none sm:max-w-md dark:ring-foreground/10 data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95",
+          className
+        )}
+        {...props}
+      >
+        {children}
+        {showCloseButton && (
+          <DialogPrimitive.Close
+            data-slot="dialog-close"
+            render={
+              <Button
+                variant="ghost"
+                className="absolute top-4 right-4 bg-secondary"
+                size="icon-sm"
+              />
+            }
+          >
+            <XIcon
+            />
+            <span className="sr-only">Schließen</span>
+          </DialogPrimitive.Close>
+        )}
+      </DialogPrimitive.Popup>
+    </DialogPortal>
+  )
+}
+
+function DialogHeader({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="dialog-header"
+      className={cn("flex flex-col gap-1.5", className)}
+      {...props}
+    />
+  )
+}
+
+function DialogFooter({
+  className,
+  showCloseButton = false,
+  children,
+  ...props
+}: React.ComponentProps<"div"> & {
+  showCloseButton?: boolean
+}) {
+  return (
+    <div
+      data-slot="dialog-footer"
+      className={cn(
+        "flex flex-col-reverse gap-2 sm:flex-row sm:justify-end",
+        className
+      )}
+      {...props}
+    >
+      {children}
+      {showCloseButton && (
+        <DialogPrimitive.Close render={<Button variant="outline" />}>
+          Schließen
+        </DialogPrimitive.Close>
+      )}
+    </div>
+  )
+}
+
+function DialogTitle({ className, ...props }: DialogPrimitive.Title.Props) {
+  return (
+    <DialogPrimitive.Title
+      data-slot="dialog-title"
+      className={cn(
+        "font-heading text-base leading-none font-medium",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function DialogDescription({
+  className,
+  ...props
+}: DialogPrimitive.Description.Props) {
+  return (
+    <DialogPrimitive.Description
+      data-slot="dialog-description"
+      className={cn(
+        "text-sm text-muted-foreground *:[a]:underline *:[a]:underline-offset-3 *:[a]:hover:text-foreground",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export {
+  Dialog,
+  DialogClose,
+  DialogContent,
+  DialogDescription,
+  DialogFooter,
+  DialogHeader,
+  DialogOverlay,
+  DialogPortal,
+  DialogTitle,
+  DialogTrigger,
+}