perf(meeting-detector): sync Command::output() in async loop blocks tokio worker

[victorlucss]

Source: Performance review for v0.2.0.

`src-tauri/src/system/meeting_detector.rs` — `Command::new("ps" / "osascript").output()` runs synchronously inside an `async fn` polling every 5 s. Each invocation blocks a tokio worker for hundreds of ms.

Recommendation: swap to `tokio::process::Command` so the spawn + waitpid is non-blocking.

Touched files: `src-tauri/src/system/meeting_detector.rs`