(bugfix) Fix proof of payment download URL to use shareID with checksum

Fixed bug where proof of payment download links used raw locationID instead
of shareID (locationID + checksum), causing link validation to fail. Added
AsyncLink component to handle async shareID generation gracefully.

Changes:
- BillEditForm: Generate shareID using generateShareId server action
- BillEditForm: Use AsyncLink to prevent broken links during async load
- AsyncLink: New reusable component for links that need async data
- Updated download URL from locationID-billID to shareID-billID format

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/app/ui/AsyncLink.tsx

@@ -0,0 +1,13 @@
+import React from "react";
+import Link from "next/link";
+
+/** Link component that can be disabled */
+export const AsyncLink: React.FC<{ href: string; children: React.ReactNode, target?: string, className?: string, disabled?: boolean }> = ({ href, children, target, className, disabled }) =>
+    disabled ? <span className={className}>{children}</span> :
+        <Link
+            href={href}
+            target={target}
+            className={className}
+        >
+            {children}
+        </Link>