Game should pause when opening the shop

[jherrflexion]

Currently, when the player opens the shop/upgrade panel, coins continue to fall and be collected in the background. For better UX, the entire game should pause while the shop is open, allowing players to make purchase decisions without pressure.

Expected behavior

• Coin spawning stops
• Falling coins freeze in place
• Game resumes when shop closes

Actual behavior

Game continues running normally while shop is open

Validation Steps (for Claude)

• Modify scripts/coin_spawner.gd to stop spawning when shop opens (listen for shop_opened signal)
• Add pause logic to coins: set process_mode or use get_tree().paused when shop opens
• Run the game (F5) and test: open shop → verify coins stop falling → close shop → verify coins resume falling smoothly
• Check that catcher doesn't collect coins while shop is open
• Verify UI remains responsive while game is paused
• Test on multiple runs to ensure consistent pause/resume behavior

Platform

• All platforms