[Foundation] Add environment configuration and secrets management

[coderabbitai]

Overview

Implement proper environment configuration for dev/staging/production.

Current State

• No .env files or examples
• No environment-specific configuration
• Hard-coded values in code

Requirements

• Create .env.example with all required variables
• Set up environment-specific configs (dev/staging/prod)
• Configure EAS secrets for builds
• Document environment setup process
• Add validation for required environment variables
• Create setup script for first-time contributors

Success Criteria

• Clear documentation for setting up environments
• No secrets committed to repository
• Easy onboarding for new developers

Priority: HIGH

Estimated Effort: 1 day

Labels: foundation, configuration, priority:high

Backlink: Related to PR #2