feat: sound notification chimes for connection lifecycle events

Audible chimes for reconnect/reconnected/reconnect-failed, self-leave,
peer join/leave, and mic/deafen toggle, completing the 4-phase
notification plan (phase 1 room/peer join+leave shipped in 673b72d).

- Per-event custom WAV overrides in Settings, with ~ expansion and live
  file-found/not-found validation; persisted on settings exit. Each event
  falls back to its embedded default chime when no custom path is set.
- Reconnect-attempt chime is edge-triggered: fires once per disconnect,
  not once per redial attempt.
- Global "enable sound notifications" toggle.

src/config.rs

@@ -36,6 +36,10 @@ impl std::fmt::Display for NetworkMode {
     }
 }
 
+fn default_true() -> bool {
+    true
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct AppConfig {
     pub input_device: String,
@@ -43,6 +47,24 @@ pub struct AppConfig {
     pub noise_gate_threshold: f32,
     #[serde(default)]
     pub network_mode: NetworkMode,
+    #[serde(default = "default_true")]
+    pub notifications_enabled: bool,
+    #[serde(default)]
+    pub custom_sound_self_join: Option<String>,
+    #[serde(default)]
+    pub custom_sound_peer_join: Option<String>,
+    #[serde(default)]
+    pub custom_sound_peer_leave: Option<String>,
+    #[serde(default)]
+    pub custom_sound_reconnect_attempt: Option<String>,
+    #[serde(default)]
+    pub custom_sound_reconnected: Option<String>,
+    #[serde(default)]
+    pub custom_sound_self_leave: Option<String>,
+    #[serde(default)]
+    pub custom_sound_mic_toggle: Option<String>,
+    #[serde(default)]
+    pub custom_sound_reconnect_failed: Option<String>,
 }
 
 impl Default for AppConfig {
@@ -52,6 +74,15 @@ impl Default for AppConfig {
             output_device: "".to_string(),
             noise_gate_threshold: 0.01,
             network_mode: NetworkMode::default(),
+            notifications_enabled: true,
+            custom_sound_self_join: None,
+            custom_sound_peer_join: None,
+            custom_sound_peer_leave: None,
+            custom_sound_reconnect_attempt: None,
+            custom_sound_reconnected: None,
+            custom_sound_self_leave: None,
+            custom_sound_mic_toggle: None,
+            custom_sound_reconnect_failed: None,
         }
     }
 }