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)

AGENTS.md

@@ -0,0 +1,5 @@
+<!-- BEGIN:nextjs-agent-rules -->
+# This is NOT the Next.js you know
+
+This version has breaking changes — APIs, conventions, and file structure may all differ from your training data. Read the relevant guide in `node_modules/next/dist/docs/` before writing any code. Heed deprecation notices.
+<!-- END:nextjs-agent-rules -->