feat: migrate schema to DATE type, add test infrastructure

- Migrate due_date/next_occurrence columns from TIMESTAMPTZ to DATE
- Update serializeRow() to distinguish DATE vs TIMESTAMPTZ serialization
- Simplify frontend date parsing (no more timezone workarounds)
- Add Vitest + Testing Library test infrastructure
- Add initial date parsing/formatting unit tests
- Update package.json with dev dependencies (vitest, testing-library, jsdom)

.dockerignore

@@ -0,0 +1,11 @@
+node_modules
+.next
+.git
+.gitignore
+*.md
+upload_to_gitea.py
+.env
+.hermes
+Dockerfile
+docker-compose.yml
+.dockerignore