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/MonthCard.tsx

@@ -2,7 +2,7 @@
 
 import { FC, useEffect, useRef } from "react";
 import { formatYearMonth } from "../lib/format";
-import { YearMonth } from "../lib/db-types";
+import { YearMonth } from '@evidencija-rezija/shared-code';
 import { formatCurrency } from "../lib/formatStrings";
 import { useTranslations } from "next-intl";