Refactor server-side modules into src/server

[EfeOzalpp]

I refactored the app’s server-side modules into a more explicit structure under src/server.

What changed

• created a src/server folder
• moved authentication-related server code under src/server/auth
• moved Prisma client setup to src/server/database.ts
• split the former src/actions.js into:
  • src/server/admin
  • src/server/general
  • src/server/shared

Notes

• shared contains read-only server queries used across the app, such as fetching users, Thursdays, groups, and semesters
• admin contains admin-oriented server actions
• general contains the remaining non-admin server helpers