feat(chat): volume control for inline audio clips

Add a per-clip volume slider plus a master volume control with a
"Universal volume" toggle in the chat header.

- ClipPlayer gains a SetVolume command; the worker remembers gain across
  clips and reapplies it to each freshly connected player.
- New config.clip_volume (universal level) and config.clip_volume_universal
  (mode toggle, default on), both persisted; old configs load at unity in
  universal mode.
- Universal on: master and per-clip sliders drive one shared level applied
  to every clip. Universal off: each clip keeps its own in-memory level and
  the master slider is inert.
- play_ready_audio applies the resolved effective gain right after Play.

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

src/config.rs

@@ -156,6 +156,14 @@ pub struct AppConfig {
     /// App-internal playback gain applied to the mixed output (1.0 = unity).
     #[serde(default = "default_volume")]
     pub output_volume: f32,
+    /// Universal playback gain for inline chat audio clips (1.0 = unity). One
+    /// level shared by every uploaded clip so the slider sticks across plays.
+    #[serde(default = "default_volume")]
+    pub clip_volume: f32,
+    /// When true, `clip_volume` governs every clip. When false, each clip keeps
+    /// its own (in-memory) level and the universal slider is inactive.
+    #[serde(default = "default_true")]
+    pub clip_volume_universal: bool,
     #[serde(default)]
     pub network_mode: NetworkMode,
     /// Presence posture for the friends idle listener (W7): invisible / normal /
@@ -312,6 +320,8 @@ impl Default for AppConfig {
             noise_gate_threshold: 0.01,
             input_volume: 1.0,
             output_volume: 1.0,
+            clip_volume: 1.0,
+            clip_volume_universal: true,
             network_mode: NetworkMode::default(),
             presence_mode: crate::presence::PresenceMode::default(),
             echo_cancellation_enabled: false,
@@ -657,23 +667,32 @@ mod tests {
         let def = AppConfig::default();
         assert_eq!(def.input_volume, 1.0);
         assert_eq!(def.output_volume, 1.0);
+        assert_eq!(def.clip_volume, 1.0);
+        assert!(def.clip_volume_universal);
 
         // 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);
+        assert_eq!(cfg_missing.clip_volume, 1.0);
+        // Configs predating the toggle default to universal mode.
+        assert!(cfg_missing.clip_volume_universal);
 
         // Explicit non-unity values are preserved across a round-trip.
         let cfg = AppConfig {
             input_volume: 1.5,
             output_volume: 0.25,
+            clip_volume: 0.7,
+            clip_volume_universal: false,
             ..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);
+        assert_eq!(round_tripped.clip_volume, 0.7);
+        assert!(!round_tripped.clip_volume_universal);
     }
 
     #[test]