Notification sounds: hand-picked mp3 palette + error/milestone cues

Replace the synthesized WAV blips with curated mp3 effects:
- error ("haiya") plays on LLM-down/timeout and save failures, debounced
  and on-transition-only, with a reassuring bilingual bubble
- milestone (Chinese fanfare) now fires every 100 words
- all other notifications draw from a rotating pop pool that never repeats
  the last sound, so batches of suggestions/tips stay lively

playPop() replaces the per-suggestion-type sound map; thread llmDown through
PetalCompanion -> useCompanion for the error cue. Old synth WAVs removed.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/components/StatusBar/SoundToggle.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from 'react'
-import { isSoundEnabled, onSoundEnabledChange, playSound, setSoundEnabled } from '../../audio/sounds'
+import { isSoundEnabled, onSoundEnabledChange, playPop, setSoundEnabled } from '../../audio/sounds'
 
 // A tiny mute toggle for Petal's cute sounds, tucked at the right of the status
 // bar. Bilingual tooltip (she reads Mandarin first), and a soft confirming pop
@@ -14,7 +14,7 @@ export function SoundToggle() {
     const next = !on
     setSoundEnabled(next)
     setOn(next)
-    if (next) playSound('pop') // a little hello when re-enabled
+    if (next) playPop() // a little hello when re-enabled
   }
 
   return (