add ISO 8601 date picker, replace all native date inputs

custom DateInput component using react-day-picker + date-fns with
Calendar popover, always displays YYYY-MM-DD regardless of browser
locale. adds shadcn Popover and Calendar components. replaces all
<input type="date"> across process-stepper, contact-detail, contact-form

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/shared/components/ui/date-input.tsx

@@ -0,0 +1,74 @@
+import { format, parse } from "date-fns";
+import { CalendarIcon } from "lucide-react";
+import { type ComponentProps, useState } from "react";
+import { Button } from "@/shared/components/ui/button";
+import { Calendar } from "@/shared/components/ui/calendar";
+import {
+	Popover,
+	PopoverContent,
+	PopoverTrigger,
+} from "@/shared/components/ui/popover";
+import { cn } from "@/shared/lib/utils";
+
+interface DateInputProps
+	extends Omit<ComponentProps<"button">, "value" | "onChange"> {
+	value: string;
+	onChange: (iso: string) => void;
+	placeholder?: string;
+}
+
+function toDate(iso: string): Date | undefined {
+	if (!iso) return undefined;
+	const d = parse(iso, "yyyy-MM-dd", new Date());
+	return Number.isNaN(d.getTime()) ? undefined : d;
+}
+
+function toISO(date: Date): string {
+	return format(date, "yyyy-MM-dd");
+}
+
+export function DateInput({
+	value,
+	onChange,
+	placeholder = "YYYY-MM-DD",
+	className,
+	id,
+	...props
+}: DateInputProps) {
+	const [open, setOpen] = useState(false);
+	const selected = toDate(value);
+
+	return (
+		<Popover open={open} onOpenChange={setOpen}>
+			<PopoverTrigger asChild>
+				<Button
+					id={id}
+					type="button"
+					variant="outline"
+					className={cn(
+						"h-9 w-full justify-start px-3 text-left font-normal",
+						!value && "text-muted-foreground",
+						className,
+					)}
+					{...props}
+				>
+					<CalendarIcon className="mr-2 size-4 shrink-0" />
+					{value || placeholder}
+				</Button>
+			</PopoverTrigger>
+			<PopoverContent className="w-auto p-0" align="start">
+				<Calendar
+					mode="single"
+					selected={selected}
+					onSelect={(day) => {
+						if (day) {
+							onChange(toISO(day));
+							setOpen(false);
+						}
+					}}
+					defaultMonth={selected}
+				/>
+			</PopoverContent>
+		</Popover>
+	);
+}