feat: add unauthenticated /share/attachment/[id] route for shared bill attachments

- Add /share/attachment/.* to public pages in middleware.ts
- Create new /share/attachment/[id] route handler for downloading attachments without authentication
- Add custom 404 page for missing shared attachments
- Update ViewBillCard component to use shared attachment route instead of authenticated route

This enables attachment downloads from shared bill pages without requiring user login.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/[locale]/share/attachment/[id]/not-found.tsx

@@ -0,0 +1,6 @@
+import { NotFoundPage } from '@/app/ui/NotFoundPage';
+ 
+const ShareAttachmentNotFound = () =>
+<NotFoundPage title="404 File Not Found" description="Could not find the requested shared attachment." />;
+
+export default ShareAttachmentNotFound;