Confetti animation on game win

[BySplashGm]

Description

The GameSummaryScreen has a winner card with a bouncing animation but no confetti. Add a confetti burst when the screen loads.

Suggested approach

Use the confetti package:

confetti: ^0.7.0

• Trigger a ConfettiController burst on initState
• Use gold + white particles matching AppColors.gold
• Auto-stop after 3 seconds

Acceptance criteria

• Confetti fires once when GameSummaryScreen mounts
• Matches the app colour palette
• Does not play again on "Play Again" without a new win