feat: Display colored error/warning messages in TTY terminals

[Kimchikilla]

Problem

Currently all error output from gws is raw JSON, which is hard to read in an interactive terminal session:

{"error":{"code":404,"message":"Not Found","reason":"notFound"}}

Proposal

Detect whether stdout/stderr is a TTY and display human-friendly colored messages when running interactively:

✗ notFound (404): Not Found          ← red bold
⚠️  Model Armor: injection detected    ← yellow bold
💡 API not enabled for your project    ← yellow bold

When piped or redirected, keep the existing JSON output for backward compatibility.

Implementation notes

• Use std::io::IsTerminal (stable since Rust 1.70) for TTY detection
• colored crate (or alternatively anstream/anstyle which are already indirect deps via clap) for ANSI colors
• Error output goes to stderr; JSON pipe output stays on stdout
• Zero behavior change for scripted/piped usage

[jpoehnelt]

Partially addressed in PR #525