fix(cli): replace polling loop with event-driven select

[frankdierolf]

Summary

Fixes #23 - CPU usage while idle due to 10ms polling loop.

• Replace 10ms polling loop with tokio::select! for event-driven operation
• Convert hotkey channel from std::sync::mpsc to tokio::sync::mpsc
• Make IPC server async by accepting connections in a background thread
• Service now has zero CPU usage when idle (was ~100 wakeups/sec)

Technical Details

The service loop previously used non-blocking calls with a 10ms sleep:

loop {
    if let Some(conn) = ipc_server.try_accept()? { ... }
    if let Ok(event) = rx.try_recv() { ... }
    sleep(Duration::from_millis(10)).await;
}

Now uses tokio::select! to block until there's actual work:

loop {
    tokio::select! {
        Some(conn) = ipc_server.accept() => { ... }
        Some(event) = hotkey_rx.recv() => { ... }
    }
}

Test plan

• Run whis start and verify it starts normally
• Check CPU usage with top or htop - should be ~0% when idle
• Test whis toggle command works via IPC
• Test direct hotkey mode works (if configured)
• Test push-to-talk mode works (if configured)