feat: notification chimes for room join / peer join / peer leave (phase 1)

Adds audible notifications for the basic membership events: a bright ascending
triad when you join a room, a soft two-note rise when another participant
joins, and a two-note fall when one leaves. Hooked into the app's UiEvent
handling (RoomJoined / PeerJoined / PeerLeft) so each is played locally by
whoever observes the event.

The chimes are short mono 16-bit WAVs generated by a committed stdlib-only
Python script (assets/sounds/generate_chimes.py) and embedded in the binary
with include_bytes!, so a deployed single binary is self-contained. The new
notify module materializes each to a temp file once, then plays it
fire-and-forget via pw-play (PipeWire-native; falls back to paplay/aplay) on a
detached thread that waits on the child -- never blocks the UI, never leaves a
zombie, and silently no-ops if no player is available.

Phase 1 of a larger plan; always-on for now. Later phases: reconnect-attempt /
reconnected sounds, a settings enable/disable toggle, per-event custom sounds,
and extra events (self-leave, mute/unmute, connection-lost).

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

assets/sounds/generate_chimes.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+"""Generate PeerSpeak's default notification chimes as small mono 16-bit WAVs.
+
+Stdlib only (wave/struct/math) so there's no build-time dependency. Re-run to
+regenerate the committed assets:
+
+    python3 assets/sounds/generate_chimes.py
+
+Each chime is a short sequence of sine notes with a soft attack (no click) and
+an exponential decay (a gentle bell-like tail), plus a quiet second harmonic so
+it reads as a "chime" rather than a flat beep. Distinct melodic shapes make the
+events recognizable without looking: ascending = arrival, descending = leaving.
+"""
+import math
+import os
+import struct
+import wave
+
+RATE = 48000          # match PeerSpeak's audio graph
+AMPLITUDE = 0.38      # clearly audible, not harsh (fraction of full scale)
+ATTACK_S = 0.006      # short fade-in to kill the start click
+DECAY = 7.0           # exponential decay rate (higher = shorter tail)
+
+# Note frequencies (Hz)
+G4, C5, E5, G5 = 392.00, 523.25, 659.25, 783.99
+
+
+def note(freq, dur):
+    """One enveloped note: fundamental + soft 2nd harmonic, attack + decay."""
+    n = int(RATE * dur)
+    out = []
+    for i in range(n):
+        t = i / RATE
+        env = math.exp(-DECAY * t)
+        if t < ATTACK_S:
+            env *= t / ATTACK_S
+        sample = math.sin(2 * math.pi * freq * t) + 0.3 * math.sin(2 * math.pi * 2 * freq * t)
+        out.append(AMPLITUDE * env * sample / 1.3)
+    return out
+
+
+def chime(notes):
+    """Concatenate (freq, dur) notes into one sample buffer."""
+    buf = []
+    for freq, dur in notes:
+        buf.extend(note(freq, dur))
+    return buf
+
+
+def write_wav(path, samples):
+    with wave.open(path, "w") as w:
+        w.setnchannels(1)
+        w.setsampwidth(2)
+        w.setframerate(RATE)
+        frames = b"".join(
+            struct.pack("<h", max(-32767, min(32767, int(s * 32767)))) for s in samples
+        )
+        w.writeframes(frames)
+    print(f"wrote {path} ({len(samples) / RATE * 1000:.0f} ms)")
+
+
+CHIMES = {
+    # You joined a room: bright ascending triad — welcoming.
+    "self-join.wav": [(C5, 0.11), (E5, 0.11), (G5, 0.30)],
+    # Someone else joined: gentle two-note rise — softer than your own join.
+    "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)],
+}
+
+
+def main():
+    here = os.path.dirname(os.path.abspath(__file__))
+    for name, notes in CHIMES.items():
+        write_wav(os.path.join(here, name), chime(notes))
+
+
+if __name__ == "__main__":
+    main()