Show retry indicator when API call is being retried

[niklasmarderx]

What

When an API call fails and is being retried (rate limit, network error), show a brief message to the user instead of silently waiting.

Why

Without feedback, users think the app is frozen. A simple Retrying (attempt 2/3)... message improves UX significantly.

How

1. Open crates/ember-llm/src/retry.rs
2. Find the retry loop
3. Before each retry attempt, print a status line to stderr:

   eprintln!("Retrying request (attempt {}/{})...", attempt, max_retries);

4. Use stderr so it doesn't interfere with piped stdout output
5. Respect a --quiet flag if one exists

Reference

The retry logic is in crates/ember-llm/src/retry.rs. Keep it simple — one line of output per retry.