feat(audio): input/output volume sliders in Settings

Adds Discord-style app-internal gain controls under each device picker:
input volume scales the captured mic (applied before the meter/gate/encode,
so it also moves the mic meter), output volume scales the mixed playback
(on top of per-peer volumes). PeerSpeak-only — no system/other-app effect.

Both persist in config (input_volume/output_volume, serde default 1.0 for
backward compat) and read live by the audio loops via f32-bit atomics, so
they take effect mid-call. Sliders apply live on drag and save on release.
The standalone mic-test monitor applies the same input gain so the test
meter reflects it. Reuses the existing apply_volume helper (unity fast-path
+ i16 saturation).

Tests: config backward-compat + round-trip for the new fields (gain math
itself is covered by the existing apply_volume tests). 65 lib tests, clippy
clean. Field-verified: input slider moves the mic-test meter.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -40,11 +40,21 @@ fn default_true() -> bool {
     true
 }
 
+fn default_volume() -> f32 {
+    1.0
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct AppConfig {
     pub input_device: String,
     pub output_device: String,
     pub noise_gate_threshold: f32,
+    /// App-internal capture gain applied to the mic before encode (1.0 = unity).
+    #[serde(default = "default_volume")]
+    pub input_volume: f32,
+    /// App-internal playback gain applied to the mixed output (1.0 = unity).
+    #[serde(default = "default_volume")]
+    pub output_volume: f32,
     #[serde(default)]
     pub network_mode: NetworkMode,
     /// Route audio through PipeWire's echo-cancel module (AEC + noise suppression).
@@ -77,6 +87,8 @@ impl Default for AppConfig {
             input_device: "".to_string(),
             output_device: "".to_string(),
             noise_gate_threshold: 0.01,
+            input_volume: 1.0,
+            output_volume: 1.0,
             network_mode: NetworkMode::default(),
             echo_cancellation_enabled: false,
             notifications_enabled: true,
@@ -142,6 +154,9 @@ mod tests {
         assert_eq!(deserialized.network_mode, NetworkMode::RelayNoDiscovery);
         assert!(!deserialized.echo_cancellation_enabled);
         assert!(deserialized.notifications_enabled);
+        // Configs predating the volume sliders must load at unity gain.
+        assert_eq!(deserialized.input_volume, 1.0);
+        assert_eq!(deserialized.output_volume, 1.0);
         assert!(deserialized.custom_sound_self_join.is_none());
         assert!(deserialized.custom_sound_peer_join.is_none());
         assert!(deserialized.custom_sound_peer_leave.is_none());
@@ -152,6 +167,31 @@ mod tests {
         assert!(deserialized.custom_sound_reconnect_failed.is_none());
     }
 
+    #[test]
+    fn test_input_output_volume_fields() {
+        // Default impl is unity gain.
+        let def = AppConfig::default();
+        assert_eq!(def.input_volume, 1.0);
+        assert_eq!(def.output_volume, 1.0);
+
+        // Missing in JSON → unity (serde default).
+        let missing = r#"{"input_device":"","output_device":"","noise_gate_threshold":0.01}"#;
+        let cfg_missing: AppConfig = serde_json::from_str(missing).unwrap();
+        assert_eq!(cfg_missing.input_volume, 1.0);
+        assert_eq!(cfg_missing.output_volume, 1.0);
+
+        // Explicit non-unity values are preserved across a round-trip.
+        let cfg = AppConfig {
+            input_volume: 1.5,
+            output_volume: 0.25,
+            ..AppConfig::default()
+        };
+        let round_tripped: AppConfig =
+            serde_json::from_str(&serde_json::to_string(&cfg).unwrap()).unwrap();
+        assert_eq!(round_tripped.input_volume, 1.5);
+        assert_eq!(round_tripped.output_volume, 0.25);
+    }
+
     #[test]
     fn test_notifications_enabled_specifically() {
         let missing_notifications = r#"{"input_device":"","output_device":"","noise_gate_threshold":0.01}"#;