Add participant assignment confetti celebration on details open

[Copilot]

Participants now get a short celebratory confetti effect when their assignment details page is shown, on both mobile and desktop layouts. The change is scoped to the participant assignment view and does not alter API or game logic.

• Assignment reveal celebration

  • Added a confetti overlay in AssignmentView rendered only when assignment details are available (currentReceiver present).
  • The animation auto-dismisses after the computed max animation window.

• Trigger semantics

  • Confetti is triggered when a receiver is shown, and guarded by receiver id to avoid duplicate replays for the same receiver while still allowing replay if receiver changes.

• Deterministic animation config

  • Implemented fixed confetti piece definitions (colors, spread, delay, duration, drift) to keep rendering predictable and lint-safe.
  • Introduced named constants for spacing and display duration for easier tuning.

useEffect(() => {
  if (!currentReceiver || lastConfettiReceiverId.current === currentReceiver.id) return

  lastConfettiReceiverId.current = currentReceiver.id
  setShowConfetti(true)
  const timer = setTimeout(() => setShowConfetti(false), CONFETTI_DISPLAY_DURATION_MS)
  return () => clearTimeout(timer)
}, [currentReceiver])

[github-actions]

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