feat: improve network configuration handling

[VolodymyrBg]

• Replace manual network selection with NETWORK_ENV variable
• Add error handling for network configuration
• Update documentation with clear network selection instructions
• Make setup process more explicit and user-friendly
• Maintain backward compatibility with existing .env files

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[TangMonk]

plz remove the NETWORK_ENV shit, it unnecessary