fix(audio): rephrase CoreAudio rate-mismatch errors for end users

The error strings flow up through stream_error and end up in user-
visible logs. Replace the internal-shaped messages ("opened at XHz
while device nominal rate is YHz") with text that names the cause,
the remediation, and the reason the fallback path is refused.

crates/qbz-player/src/player/mod.rs

@@ -1251,7 +1251,7 @@ impl Player {
                                     == AudioBackendType::SystemDefault
                                 {
                                     log::error!(
-                                        "macOS CoreAudio init failed: {} — refusing unsafe legacy CPAL fallback",
+                                        "Could not start macOS audio output — {}. Not falling back to the legacy CPAL path because it would either play at the wrong speed (shared mode) or silently drop Exclusive Mode.",
                                         e
                                     );
                                     state.set_current_device(None);