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/Companion/PetalCompanion.tsx

@@ -7,6 +7,7 @@ import { COMPANIONS, DEFAULT_COMPANION } from './companions'
 interface Props {
   wordCount: number
   saveStatus: SaveStatus
+  llmDown: boolean
   editTick: number
   acceptTick: number
   text: string
@@ -27,10 +28,11 @@ const STORAGE_KEY = 'petal.companion'
 // useCompanion and shows a Mandarin-first speech bubble for cheers, tips, and
 // break reminders. Clicking the mascot opens a picker to switch companions
 // (the choice persists in localStorage).
-export function PetalCompanion({ wordCount, saveStatus, editTick, acceptTick, text }: Props) {
+export function PetalCompanion({ wordCount, saveStatus, llmDown, editTick, acceptTick, text }: Props) {
   const { mood, bubble, dismiss, holdBubble, releaseBubble } = useCompanion({
     wordCount,
     saveStatus,
+    llmDown,
     editTick,
     acceptTick,
     text,