Added 'public_ip' as an option for periphery configuration

[KapralMark]

Summary

Adds a public_ip option to Periphery (env: PERIPHERY_PUBLIC_IP) that
sets the host address explicitly instead of resolving it via OpenDNS.

Why

Periphery discovers the host public IP by querying the OpenDNS resolvers.
On air-gapped/offline networks that query can't be executed, and it causes the 'Server'
page to show Unknown IP for the machine.

Changes

File	Change
client/core/rs/src/entities/config/periphery.rs	added support for public_ip
bin/periphery/src/config.rs	reads public_ip environment value, falling back to the config value
bin/periphery/src/state.rs	tries to get public_ip instead of querying the OpenDNS resolvers
config/periphery.config.toml	documented public_ip

Behavior

• Set -> the DNS lookup is skipped and the set value is used.
• Empty, whitespace, or unset -> existing OpenDNS behavior, unchanged.

Quick note: This change is cosmetic only. The public_ip value isn't used in any place other than the 'Server' tab.

Testing

• cargo build - succeeds.
• cargo test - succeeds.
• rustfmt --check - succeeds.
• With PERIPHERY_PUBLIC_IP set as an environment variable, the Server page shows the configured
  value and doesn't try to send an OpenDNS query.
• With it unset, periphery uses OpenDNS, same as before.