chore: remove dead code flagged by knip

Remove 19 unused source files (no importers anywhere in src/):
- src/actions/admin-permissions.ts, admin-radio.ts, admin-user-edit.ts,
  admin-users.ts (functionality lives in @/actions/users and
  @/actions/permissions)
- src/components/admin/confirm-action.tsx, page-header.tsx
- src/components/motion-elements.tsx
- src/lib/format-date.ts
- src/lib/catalog-categories/* (incl. re-export barrel)
- src/lib/foundation/{index,database,middleware,validation}.ts (errors/action
  kept, still imported directly)
- src/lib/services/imager/{avatar-renderer,memory-cache}.ts
- src/lib/services/nitro-assets.ts

Update admin-operations-contract test to drop the two removed action-file
gates (their permission coverage already exists in users.ts/permissions.ts).

Add knip.json for repeatable dead-code audits.

Verified: tsc --noEmit clean, next build succeeds, full test suite green
(301/301).

src/components/admin/confirm-action.tsx

@@ -1,40 +0,0 @@
-"use client";
-
-import type { ReactNode } from "react";
-
-export function ConfirmAction({
-	message,
-	action,
-	userId,
-	className,
-	style,
-	children,
-}: {
-	message: string;
-	action: string;
-	userId: number;
-	className?: string;
-	style?: React.CSSProperties;
-	children: ReactNode;
-}) {
-	return (
-		<form
-			action="/api/admin/users/actions"
-			method="post"
-			style={{ display: "inline-block" }}
-		>
-			<input type="hidden" name="userId" value={userId} />
-			<input type="hidden" name="action" value={action} />
-			<button
-				type="submit"
-				className={className}
-				style={style}
-				onClick={(e) => {
-					if (!confirm(message)) e.preventDefault();
-				}}
-			>
-				{children}
-			</button>
-		</form>
-	);
-}

src/components/admin/page-header.tsx

@@ -1,29 +0,0 @@
-import type { LucideIcon } from "lucide-react";
-
-interface AdminPageHeaderProps {
-	icon: LucideIcon;
-	title: string;
-	subtitle: string;
-}
-
-export function AdminPageHeader({
-	icon: Icon,
-	title,
-	subtitle,
-}: AdminPageHeaderProps) {
-	return (
-		<div className="flex items-center gap-3 mb-6">
-			<div className="w-10 h-10 rounded-xl bg-gradient-to-br from-[var(--admin-accent)]/20 to-[var(--admin-accent)]/5 grid place-items-center">
-				<Icon size={20} className="text-[var(--admin-accent)]" />
-			</div>
-			<div>
-				<h1 className="m-0 text-xl font-extrabold text-[var(--admin-text)]">
-					{title}
-				</h1>
-				<p className="m-0 text-xs text-[var(--admin-text-muted)] mt-0.5">
-					{subtitle}
-				</p>
-			</div>
-		</div>
-	);
-}