feat : add authenticated admin access boundaries

[SYLVIANNORUKA]

##Summary
-Management routes (/passes, /guilds, /members, /activity, /settings) are now protected by a shared AdminGuard component — unauthenticated visitors see a clear access-denied screen instead of the raw page content
-Added getSession(): Session | null to lib/auth/session.ts as the single pluggable auth entry point — swapping in real auth (next-auth, SIWE, JWT) only requires changing this one function
-Added MOCK_ADMIN_MODE flag driven by NEXT_PUBLIC_MOCK_ADMIN=true in .env.local so the guard can be bypassed locally without hard-coding credentials
-AdminGuard lives inside DashboardLayout, so the boundary is inherited by every management page with zero per-page duplication
-Updated useSession() to return Session | null (null = unauthenticated) and updated the three pages that called it (passes, members, settings) to handle the null case safely.
closes #23

[Lakes41]

Good Job, Do resolve conflicts @SYLVIANNORUKA

[Lakes41]

@SYLVIANNORUKA Do resolve conflicts, I might have to take you off when it 24 hours