Add stage 2 easter egg flow

[xh1410]

Closes #5.
Closes #6.
Closes #7.
Closes #8.

Summary

• add a second button and track a six-click alternating sequence across both buttons
• unlock the celebration after the sequence with a yellow background, confetti, and Ты выйграл
• add a subtle Пасхалка ждет тебя! hint that triggers an explosion easter egg after three activations

Testing

• Ran a local DOM interaction check that simulated the six-click alternation and verified:
  • Alternation sequence: 6 / 6
  • celebration message visible with Ты выйграл
  • yellow state-win body class
  • 24 confetti pieces created
  • easter hint creates active explosion with 16 sparks
• git diff --check