Fix Google signup completion tracking - #121

Summary

• return whether Google auth created a new persisted account
• emit signup_completed and sign_up only for new Google accounts
• keep returning Google users classified as login

Verification

• RED: backend response lacked is_new_user; desktop/mobile emitted login for a new Google account
• GREEN: 399 backend tests passed
• GREEN: 114 Playwright tests passed, 6 expected skips
• GREEN: 119 static regressions plus security, syntax, public-shell sync, and performance budgets
• GA4 readback: signup_completed is already configured as a key event
• production reconciliation: the previously untracked account was Google-created