Implement sessionStore using Zustand

[mhd-hi]

Instead of using react context, use zustand

Session store responsibilities:

• Manage sessions and their associated course instances

sessionKeys: Record<string, Session>

type Session = {
  key: string; ("2025-AUTOMNE" ou simialire)
  year: number;
  trimester: Trimester;
  courseIdKeys: number[]; // References to courses in courseStore
};