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/toggle.tsx

@@ -0,0 +1,44 @@
+"use client"
+
+import { Toggle as TogglePrimitive } from "@base-ui/react/toggle"
+import { cva, type VariantProps } from "class-variance-authority"
+import { cn } from "@/lib/utils"
+
+const toggleVariants = cva(
+  "group/toggle inline-flex items-center justify-center gap-1 rounded-3xl text-sm font-medium whitespace-nowrap transition-colors outline-none hover:bg-muted hover:text-foreground focus-visible:border-ring focus-visible:ring-[3px] focus-visible:ring-ring/30 disabled:pointer-events-none disabled:opacity-50 aria-invalid:border-destructive aria-invalid:ring-destructive/20 aria-pressed:bg-muted dark:aria-invalid:ring-destructive/40 [&_svg]:pointer-events-none [&_svg]:shrink-0 [&_svg:not([class*='size-'])]:size-4",
+  {
+    variants: {
+      variant: {
+        default: "bg-transparent",
+        outline: "border border-input bg-transparent hover:bg-muted",
+      },
+      size: {
+        default:
+          "h-9 min-w-9 px-3 has-data-[icon=inline-end]:pr-2.5 has-data-[icon=inline-start]:pl-2.5",
+        sm: "h-8 min-w-8 px-3 has-data-[icon=inline-end]:pr-2 has-data-[icon=inline-start]:pl-2",
+        lg: "h-10 min-w-10 px-4 has-data-[icon=inline-end]:pr-3 has-data-[icon=inline-start]:pl-3",
+      },
+    },
+    defaultVariants: {
+      variant: "default",
+      size: "default",
+    },
+  }
+)
+
+function Toggle({
+  className,
+  variant = "default",
+  size = "default",
+  ...props
+}: TogglePrimitive.Props & VariantProps<typeof toggleVariants>) {
+  return (
+    <TogglePrimitive
+      data-slot="toggle"
+      className={cn(toggleVariants({ variant, size, className }))}
+      {...props}
+    />
+  )
+}
+
+export { Toggle, toggleVariants }