Oxygen Warning, Death Penalty, and Balance Update

Hi divers,

This build focuses on making each dive clearer, tenser, and more meaningful.

## Key Changes

### Stronger Low-Oxygen Feedback
Low oxygen is now easier to notice during a dive.

- Added a red pulsing warning around the oxygen display
- Added a darker screen effect when oxygen gets dangerously low
- Increased oxygen bubble size for better readability

### New Death Penalty
Failed dives now have a clearer cost.

- Equipment upgrades are reset after death
- Current coins are reduced by 50%
- The base summary now shows death penalty details

### Restart Flow Update
The pause menu’s base return option now restarts at base instead of treating the action as a normal failed dive.

### Balance Adjustment
Shallow coin pickup value has been reduced from 10 to 5.

This slows down early progression and makes upgrade choices more important.

### Upgrade Menu Layout
The upgrade menu size has been adjusted for cleaner display across different window sizes.