Move shadcn directories to client shared folder + update shadcn docs a bit (#551)

template/app/src/client/components/ui/checkbox.tsx

@@ -0,0 +1,28 @@
+import * as CheckboxPrimitive from "@radix-ui/react-checkbox";
+import { Check } from "lucide-react";
+import * as React from "react";
+
+import { cn } from "../../utils";
+
+const Checkbox = React.forwardRef<
+  React.ElementRef<typeof CheckboxPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof CheckboxPrimitive.Root>
+>(({ className, ...props }, ref) => (
+  <CheckboxPrimitive.Root
+    ref={ref}
+    className={cn(
+      "border-primary focus-visible:ring-ring data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground peer h-4 w-4 shrink-0 rounded-sm border shadow focus-visible:outline-none focus-visible:ring-1 disabled:cursor-not-allowed disabled:opacity-50",
+      className,
+    )}
+    {...props}
+  >
+    <CheckboxPrimitive.Indicator
+      className={cn("flex items-center justify-center text-current")}
+    >
+      <Check className="h-4 w-4" />
+    </CheckboxPrimitive.Indicator>
+  </CheckboxPrimitive.Root>
+));
+Checkbox.displayName = CheckboxPrimitive.Root.displayName;
+
+export { Checkbox };