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/core/messages.rs

@@ -23,6 +23,7 @@ pub enum UiEvent {
     RoomLeft,
     PeerJoined { id: EndpointId, state: PeerState },
     PeerLeft { id: EndpointId },
+    PeerConnectionFailed { id: EndpointId },
     PeerUpdated { id: EndpointId, state: PeerState },
     /// Audio link to a peer is being (re)established — show a connecting state.
     PeerConnecting { id: EndpointId },