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/ui/HomePage.tsx

@@ -1,7 +1,7 @@
 import { fetchAllLocations } from '@/app/lib/actions/locationActions';
 import { fetchAvailableYears } from '@/app/lib/actions/monthActions';
 import { getUserSettings } from '@/app/lib/actions/userSettingsActions';
-import { BillingLocation, YearMonth } from '@/app/lib/db-types';
+import { BillingLocation, YearMonth } from '@evidencija-rezija/shared-code';
 import { FC } from 'react';
 import { MonthLocationList } from '@/app/ui/MonthLocationList';