fix: memoize session to prevent race condition on dashboard

[ephraimduncan]

Summary

• Wrap getSession() with React's cache() to memoize within a request
• Update TRPC context to use shared getSession() instead of calling auth.api.getSession() directly

Problem

Sporadic TRPCError: UNAUTHORIZED errors on /dashboard due to race condition:

1. Layout calls getSession() → cookie cache valid → passes
2. TRPC context calls auth.api.getSession() → cookie cache expires → fails
3. protectedProcedure throws UNAUTHORIZED

Solution

Memoize getSession() so both layout and TRPC share the same session result within each request.

Test plan

• Deploy and monitor for UNAUTHORIZED errors on /dashboard
• Verify dashboard loads correctly for authenticated users