Upgrade @clerk/clerk-expo to @clerk/expo@3 canary

[jacekradko]

Summary

• Replaces @clerk/clerk-expo with @clerk/expo@3.0.0-canary.v20260213015452
• Runs @clerk/upgrade codemods to apply all Core 3 breaking changes (import renames, <Protect> → <Show>, @clerk/types → @clerk/shared/types)
• Updates remaining manual references in _layout.tsx and README.md

Test plan

• App boots without errors on iOS/Android
• Sign-in and sign-up flows work end to end
• Sign-out works
• Protected routes redirect unauthenticated users