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/MonthLocationList.tsx

@@ -7,7 +7,7 @@ import { MonthCard } from "./MonthCard";
 import Pagination from "./Pagination";
 import { LocationCard } from "./LocationCard";
 import { PrintButton } from "./PrintButton";
-import { BillingLocation, UserSettings, YearMonth } from "../lib/db-types";
+import { BillingLocation, UserSettings, YearMonth } from '@evidencija-rezija/shared-code';
 import { useRouter, useSearchParams } from "next/navigation";
 import { ToastContainer, toast } from 'react-toastify';
 import 'react-toastify/dist/ReactToastify.css';