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/package.json

@@ -9,6 +9,7 @@
     "seed": "node -r dotenv/config ./scripts/seed.js"
   },
   "dependencies": {
+    "@evidencija-rezija/shared-code": "^1.0.0",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@heroicons/react": "^2.0.18",