Improve error message when daemon is not running

[sprklai]

Description

When the daemon isn't running, the CLI shows a generic connection refused error. This should be detected and replaced with an actionable message: "Cannot connect to daemon at localhost:18981. Start it with: zenii daemon start"

Files to modify

• crates/zenii-cli/src/main.rs — detect connection refused errors and show friendly message

Acceptance criteria

• Connection refused → "Cannot connect to daemon. Start it with: zenii daemon start"
• Timeout → "Daemon not responding. Check if it's running: zenii daemon status"
• Other HTTP errors still show detailed error info
• Works for all CLI commands that talk to the daemon

Difficulty: Easy (30-60 min)

This project is written in Rust. See CONTRIBUTING.md for setup instructions.