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/field.tsx

@@ -0,0 +1,238 @@
+"use client"
+
+import { useMemo } from "react"
+import { cva, type VariantProps } from "class-variance-authority"
+import { cn } from "@/lib/utils"
+
+import { Label } from "@/components/ui/label"
+import { Separator } from "@/components/ui/separator"
+
+function FieldSet({ className, ...props }: React.ComponentProps<"fieldset">) {
+  return (
+    <fieldset
+      data-slot="field-set"
+      className={cn(
+        "flex flex-col gap-6 has-[>[data-slot=checkbox-group]]:gap-3 has-[>[data-slot=radio-group]]:gap-3",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldLegend({
+  className,
+  variant = "legend",
+  ...props
+}: React.ComponentProps<"legend"> & { variant?: "legend" | "label" }) {
+  return (
+    <legend
+      data-slot="field-legend"
+      data-variant={variant}
+      className={cn(
+        "mb-3 font-medium data-[variant=label]:text-sm data-[variant=legend]:text-base",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldGroup({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="field-group"
+      className={cn(
+        "group/field-group @container/field-group flex w-full flex-col gap-7 data-[slot=checkbox-group]:gap-3 *:data-[slot=field-group]:gap-4",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+const fieldVariants = cva(
+  "group/field flex w-full gap-3 data-[invalid=true]:text-destructive",
+  {
+    variants: {
+      orientation: {
+        vertical: "flex-col *:w-full [&>.sr-only]:w-auto",
+        horizontal:
+          "flex-row items-center has-[>[data-slot=field-content]]:items-start *:data-[slot=field-label]:flex-auto has-[>[data-slot=field-content]]:[&>[role=checkbox],[role=radio]]:mt-px",
+        responsive:
+          "flex-col *:w-full @md/field-group:flex-row @md/field-group:items-center @md/field-group:*:w-auto @md/field-group:has-[>[data-slot=field-content]]:items-start @md/field-group:*:data-[slot=field-label]:flex-auto [&>.sr-only]:w-auto @md/field-group:has-[>[data-slot=field-content]]:[&>[role=checkbox],[role=radio]]:mt-px",
+      },
+    },
+    defaultVariants: {
+      orientation: "vertical",
+    },
+  }
+)
+
+function Field({
+  className,
+  orientation = "vertical",
+  ...props
+}: React.ComponentProps<"div"> & VariantProps<typeof fieldVariants>) {
+  return (
+    <div
+      role="group"
+      data-slot="field"
+      data-orientation={orientation}
+      className={cn(fieldVariants({ orientation }), className)}
+      {...props}
+    />
+  )
+}
+
+function FieldContent({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="field-content"
+      className={cn(
+        "group/field-content flex flex-1 flex-col gap-1 leading-snug",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldLabel({
+  className,
+  ...props
+}: React.ComponentProps<typeof Label>) {
+  return (
+    <Label
+      data-slot="field-label"
+      className={cn(
+        "group/field-label peer/field-label flex w-fit gap-2 leading-snug group-data-[disabled=true]/field:opacity-50 has-data-checked:bg-input/30 has-[>[data-slot=field]]:rounded-2xl has-[>[data-slot=field]]:border has-[>[data-slot=field]]:not-has-[:disabled,[data-disabled]]:hover:bg-input/40 has-[>[data-slot=field]]:has-[:focus-visible]:border-ring has-[>[data-slot=field]]:has-[:focus-visible]:ring-3 has-[>[data-slot=field]]:has-[:focus-visible]:ring-ring/50 *:data-[slot=field]:p-4",
+        "has-[>[data-slot=field]]:w-full has-[>[data-slot=field]]:flex-col",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldTitle({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="field-label"
+      className={cn(
+        "flex w-fit items-center gap-2 text-sm font-medium group-data-[disabled=true]/field:opacity-50",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldDescription({ className, ...props }: React.ComponentProps<"p">) {
+  return (
+    <p
+      data-slot="field-description"
+      className={cn(
+        "text-left text-sm leading-normal font-normal text-muted-foreground group-has-data-horizontal/field:text-balance [[data-variant=legend]+&]:-mt-1.5",
+        "last:mt-0 nth-last-2:-mt-1",
+        "[&>a]:underline [&>a]:underline-offset-4 [&>a:hover]:text-primary",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+function FieldSeparator({
+  children,
+  className,
+  ...props
+}: React.ComponentProps<"div"> & {
+  children?: React.ReactNode
+}) {
+  return (
+    <div
+      data-slot="field-separator"
+      data-content={!!children}
+      className={cn(
+        "relative -my-2 h-5 text-sm group-data-[variant=outline]/field-group:-mb-2",
+        className
+      )}
+      {...props}
+    >
+      <Separator className="absolute inset-0 top-1/2" />
+      {children && (
+        <span
+          className="relative mx-auto block w-fit bg-background px-2 text-muted-foreground"
+          data-slot="field-separator-content"
+        >
+          {children}
+        </span>
+      )}
+    </div>
+  )
+}
+
+function FieldError({
+  className,
+  children,
+  errors,
+  ...props
+}: React.ComponentProps<"div"> & {
+  errors?: Array<{ message?: string } | undefined>
+}) {
+  const content = useMemo(() => {
+    if (children) {
+      return children
+    }
+
+    if (!errors?.length) {
+      return null
+    }
+
+    const uniqueErrors = [
+      ...new Map(errors.map((error) => [error?.message, error])).values(),
+    ]
+
+    if (uniqueErrors?.length == 1) {
+      return uniqueErrors[0]?.message
+    }
+
+    return (
+      <ul className="ml-4 flex list-disc flex-col gap-1">
+        {uniqueErrors.map(
+          (error, index) =>
+            error?.message && <li key={index}>{error.message}</li>
+        )}
+      </ul>
+    )
+  }, [children, errors])
+
+  if (!content) {
+    return null
+  }
+
+  return (
+    <div
+      role="alert"
+      data-slot="field-error"
+      className={cn("text-sm font-normal text-destructive", className)}
+      {...props}
+    >
+      {content}
+    </div>
+  )
+}
+
+export {
+  Field,
+  FieldLabel,
+  FieldDescription,
+  FieldError,
+  FieldGroup,
+  FieldLegend,
+  FieldSeparator,
+  FieldSet,
+  FieldContent,
+  FieldTitle,
+}