cli: add --json flag to device get command

[martinsander00]

Resolves: #3036

Summary

• Add --json and --json-compact flags to doublezero device get --code <code> for machine-readable output
• Follows the same pattern used by all existing list commands (device list, user list, etc.)
• Introduces a DeviceGetDisplay struct with proper serde serialization (pubkeys as base58 strings, network prefixes as CIDR strings)

Lines of Code

Section	Added	Removed
cli	+111	-28
tests	+47	-4

Testing Verification

• Added test_cli_device_get_json unit test validating JSON output parses correctly and contains expected field values
• Existing test_cli_device_get updated and continues to pass, confirming no regression in text output