Enhance coin collection system with improved spawning and smoke trail effects (#26)

- Fix coin spawning throughout entire descent to 1500ft altitude
- Increase coin spawn frequency (60% chance vs 30%) and quantity (2-5 vs 1-3)
- Extend spawn range for better distribution across descent path
- Fix coin cleanup bug that caused instant disappearing
- Add colored smoke trail particle effect that activates when collecting coins
- Implement 8-color cycling system for visual feedback
- Add SmokeTrail CPUParticles2D node to Player with proper timer management

Improves gameplay flow by ensuring coins are available throughout the entire fall
and provides satisfying visual feedback for collection achievements.

Author: alanops