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-group.tsx

@@ -0,0 +1,89 @@
+"use client"
+
+import * as React from "react"
+import { Toggle as TogglePrimitive } from "@base-ui/react/toggle"
+import { ToggleGroup as ToggleGroupPrimitive } from "@base-ui/react/toggle-group"
+import { type VariantProps } from "class-variance-authority"
+import { cn } from "@/lib/utils"
+
+import { toggleVariants } from "@/components/ui/toggle"
+
+const ToggleGroupContext = React.createContext<
+  VariantProps<typeof toggleVariants> & {
+    spacing?: number
+    orientation?: "horizontal" | "vertical"
+  }
+>({
+  size: "default",
+  variant: "default",
+  spacing: 2,
+  orientation: "horizontal",
+})
+
+function ToggleGroup({
+  className,
+  variant,
+  size,
+  spacing = 2,
+  orientation = "horizontal",
+  children,
+  ...props
+}: ToggleGroupPrimitive.Props &
+  VariantProps<typeof toggleVariants> & {
+    spacing?: number
+    orientation?: "horizontal" | "vertical"
+  }) {
+  return (
+    <ToggleGroupPrimitive
+      data-slot="toggle-group"
+      data-variant={variant}
+      data-size={size}
+      data-spacing={spacing}
+      data-orientation={orientation}
+      style={{ "--gap": spacing } as React.CSSProperties}
+      className={cn(
+        "group/toggle-group flex w-fit flex-row items-center gap-[--spacing(var(--gap))] data-[spacing=0]:data-[variant=outline]:rounded-3xl data-vertical:flex-col data-vertical:items-stretch",
+        className
+      )}
+      {...props}
+    >
+      <ToggleGroupContext.Provider
+        value={{ variant, size, spacing, orientation }}
+      >
+        {children}
+      </ToggleGroupContext.Provider>
+    </ToggleGroupPrimitive>
+  )
+}
+
+function ToggleGroupItem({
+  className,
+  children,
+  variant = "default",
+  size = "default",
+  ...props
+}: TogglePrimitive.Props & VariantProps<typeof toggleVariants>) {
+  const context = React.useContext(ToggleGroupContext)
+
+  return (
+    <TogglePrimitive
+      data-slot="toggle-group-item"
+      data-variant={context.variant || variant}
+      data-size={context.size || size}
+      data-spacing={context.spacing}
+      className={cn(
+        "shrink-0 group-data-[spacing=0]/toggle-group:rounded-none group-data-[spacing=0]/toggle-group:px-3 group-data-[spacing=0]/toggle-group:shadow-none focus:z-10 focus-visible:z-10 group-data-[spacing=0]/toggle-group:has-data-[icon=inline-end]:pr-2.5 group-data-[spacing=0]/toggle-group:has-data-[icon=inline-start]:pl-2.5 group-data-horizontal/toggle-group:data-[spacing=0]:first:rounded-l-3xl group-data-vertical/toggle-group:data-[spacing=0]:first:rounded-t-3xl group-data-horizontal/toggle-group:data-[spacing=0]:last:rounded-r-3xl group-data-vertical/toggle-group:data-[spacing=0]:last:rounded-b-3xl data-[state=on]:bg-muted group-data-horizontal/toggle-group:data-[spacing=0]:data-[variant=outline]:border-l-0 group-data-vertical/toggle-group:data-[spacing=0]:data-[variant=outline]:border-t-0 group-data-horizontal/toggle-group:data-[spacing=0]:data-[variant=outline]:first:border-l group-data-vertical/toggle-group:data-[spacing=0]:data-[variant=outline]:first:border-t",
+        toggleVariants({
+          variant: context.variant || variant,
+          size: context.size || size,
+        }),
+        className
+      )}
+      {...props}
+    >
+      {children}
+    </TogglePrimitive>
+  )
+}
+
+export { ToggleGroup, ToggleGroupItem }