Introduces a schema version field to GameState. Saves without the
current schema version are flagged on load (showing a modal prompting
reset or proceed), and cloud saves from outdated clients are rejected
server-side. Removes all backfill code now that outdated saves are
handled via the reset flow instead.
New POST /game/reset endpoint creates a fresh save for players who
choose to reset. Save version and current schema version are displayed
in the sidebar below the app version.
