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.

assets/sounds/generate_chimes.py

@@ -66,6 +66,16 @@ CHIMES = {
     "peer-join.wav": [(G4, 0.13), (C5, 0.28)],
     # Someone left: two-note fall — the mirror of an arrival.
     "peer-leave.wav": [(C5, 0.13), (G4, 0.28)],
+    # Reconnecting to peer: soft single Bb4 note.
+    "reconnect-attempt.wav": [(466.16, 0.20)],
+    # Reconnected to peer: bright two-note rise (C5 -> G5).
+    "reconnected.wav": [(C5, 0.10), (G5, 0.25)],
+    # Left a room: descending triad.
+    "self-leave.wav": [(G5, 0.10), (E5, 0.10), (C5, 0.25)],
+    # Mute/unmute toggle: tiny clean blip.
+    "mic-toggle.wav": [(E5, 0.08)],
+    # Reconnect gave up: disappointing low two-note fall.
+    "reconnect-failed.wav": [(C5, 0.15), (349.23, 0.30)],
 }