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/lib/actions/billActions.ts

@@ -10,7 +10,7 @@ import { gotoHomeWithMessage } from './navigationActions';
 import { getTranslations, getLocale } from "next-intl/server";
 import { IntlTemplateFn } from '@/app/i18n';
 import { unstable_noStore, revalidatePath } from 'next/cache';
-import { extractShareId, validateShareChecksum } from '../shareChecksum';
+import { extractShareId, validateShareChecksum } from '@evidencija-rezija/shared-code';
 import { validatePdfFile } from '../validators/pdfValidator';
 import { checkUploadRateLimit } from '../uploadRateLimiter';