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/LocationEditForm.tsx

@@ -2,7 +2,7 @@
 
 import { TrashIcon, ExclamationTriangleIcon, ClockIcon, EnvelopeIcon, CheckCircleIcon, PencilSquareIcon, XCircleIcon } from "@heroicons/react/24/outline";
 import { FC, useState } from "react";
-import { BillingLocation, UserSettings, YearMonth, EmailStatus } from "../lib/db-types";
+import { BillingLocation, UserSettings, YearMonth, EmailStatus } from '@evidencija-rezija/shared-code';
 import { updateOrAddLocation } from "../lib/actions/locationActions";
 import { useFormState } from "react-dom";
 import Link from "next/link";