feat(audio): add audio_output_device to Settings

Device::get_id() string; None = system default (unchanged behavior).
#[serde(default)] so an existing settings.json without this key still
deserializes.

src-tauri/src/models.rs

@@ -213,6 +213,10 @@ pub struct Settings {
     pub pst_last_path: Option<String>,
     #[serde(default)]
     pub pst_auto_sync: bool,
+    // Audio capture device override (FR-CAP-1). `Device::get_id()` string;
+    // None = system default render device (loopback capture has no mic path).
+    #[serde(default)]
+    pub audio_output_device: Option<String>,
 }
 
 // ---- Sync (ADR-0010) ----