Make the web app consume the live mount contract

Add the first control-plane UI over the verified backend seam so exports,
issued DAV credentials, and Finder instructions can be exercised from the
running stack.

apps/web/app/copy-field.tsx

@@ -0,0 +1,29 @@
+"use client";
+
+import { Button } from "@betternas/ui/button";
+import { Code } from "@betternas/ui/code";
+import { useState } from "react";
+import styles from "./copy-field.module.css";
+
+export function CopyField({ label, value }: { label: string; value: string }) {
+  const [copied, setCopied] = useState(false);
+
+  return (
+    <div className={styles.field}>
+      <div className={styles.header}>
+        <span className={styles.label}>{label}</span>
+        <Button
+          className={styles.button}
+          onClick={async () => {
+            await navigator.clipboard.writeText(value);
+            setCopied(true);
+            window.setTimeout(() => setCopied(false), 1500);
+          }}
+        >
+          {copied ? "Copied" : "Copy"}
+        </Button>
+      </div>
+      <Code className={styles.value}>{value}</Code>
+    </div>
+  );
+}