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>

middleware.ts

@@ -10,7 +10,7 @@ import { locales, defaultLocale } from '@/app/i18n';
 import { Session } from 'next-auth';
 
 // http://localhost:3000/share/location/675c41b227d0df76a35f106e
-const publicPages = ['/terms', '/policy', '/login', '/share/location/.*', '/share/bill/.*'];
+const publicPages = ['/terms', '/policy', '/login', '/share/location/.*', '/share/bill/.*', '/share/attachment/.*'];
  
 const intlMiddleware = createIntlMiddleware({
   locales,