feat: add Einnahmen Kategorien management page with CRUD functionality

- Implemented a new route for managing Einnahmen Kategorien.
- Added auto-seeding of default Einnahmen Kategorien if none exist.
- Integrated category usage tracking to prevent deletion of in-use categories.
- Enhanced Einnahmen page to link to the new Kategorien management.
- Updated Prisma schema and seed script to include default categories.
- Added a modal for detailed view of Einnahmen by category and month.
- Refactored existing Einnahmen page to accommodate new category structure.
- Introduced PostCSS configuration for Tailwind CSS support.
- Created a new migration to update existing category labels in the database.
- Added TypeScript configuration for stricter type checking.
- Set up Vite configuration for improved development experience with React Router.

react-router.config.js

@@ -0,0 +1,3 @@
+export default {
+    ssr: true,
+};