fix(auth): add helpful error when PKCE code verifier is missing

[mandarini]

Summary

• Adds AuthPKCECodeVerifierMissingError with actionable guidance for users
• Validates code verifier presence before making the token exchange request
• Only triggers for PKCE flow clients to maintain backward compatibility

Problem

Users calling exchangeCodeForSession() when the code verifier is missing (common when email link is opened in different browser/device, or SSR apps without cookie storage) receive an unhelpful server error: "both auth code and code verifier should be non-empty".

Solution

Detect the missing code verifier client-side and throw a helpful error:

  > PKCE code verifier not found in storage. This can happen if the auth flow was initiated in a different browser or device, or if the storage was cleared. For server-side auth, ensure you are using @supabase/ssr with cookie-based storage.

Closes #950

[coveralls]

coverage: 95.367% (+14.1%) from 81.223%
when pulling 50d9c26 on fix/auth-pkce-code-verifier-missing-error
into 139a745 on master.