refactor: migrate web-app to use shared-code package

Update web-app to use @evidencija-rezija/shared-code for common types
and utilities instead of maintaining duplicate copies.

Changes:
- Add shared-code dependency to package.json
- Update all imports across 35+ files to use @evidencija-rezija/shared-code
- Remove duplicate db-types.ts and shareChecksum.ts files

This ensures type consistency between web-app and email-worker and
reduces maintenance burden.

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

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

web-app/app/[locale]/share/proof-of-payment/combined/[id]/route.tsx

@@ -1,7 +1,7 @@
 import { getDbClient } from '@/app/lib/dbClient';
-import { BillingLocation } from '@/app/lib/db-types';
+import { BillingLocation } from '@evidencija-rezija/shared-code';
 import { notFound } from 'next/navigation';
-import { extractShareId, validateShareChecksum } from '@/app/lib/shareChecksum';
+import { extractShareId, validateShareChecksum } from '@evidencija-rezija/shared-code';
 
 export async function GET(request: Request, { params: { id } }: { params: { id: string } }) {
     const shareId = id;