Title screen previews all seven chapters; social login working

Open the game and the title screen shows the seven chapters of Song of Larithea, not just the current run. New players see what they're stepping into; returning players see where they are in the arc. The shift is from a start screen that surfaced only the immediate next beat to one that previews the whole road.

Social login is wired end-to-end and working from the start screen. One tap signs you in — no email-password flow, no separate sign-up step. The auth plumbing has been landing across the last two weeks (multi-identity linking on May 1, Apple sign-in cleanup on May 2); today it's all the way through.