Add PHP server version: leaderboard, identity, menu screens, lore

- php/: MySQL-backed (via config.php, gitignored) top-5 solutions per
  level, cookie identity with rename, server-side progress for "Продолжить"
- Menu screens: Играть / Продолжить / Предыстория / Настройки, plus
  placeholder start/end screens for a future intro/outro
- Lore screen: EXO:GRID backstory text (6 epochs, no gameplay changes yet)
- levels.js/solver.js copied in as-is

php/.gitignore

@@ -0,0 +1 @@
+data/