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/tips.ts

@@ -48,8 +48,16 @@ export const WELCOME_BACK: Line = {
   en: 'Welcome back ✨ let’s keep going!',
 }
 
-// Word-count milestones worth a little cheer.
-export const MILESTONES = [50, 100, 250, 500, 1000, 2000]
+// Gentle "haiya, something went wrong" lines — paired with the error sound when
+// the LLM is unreachable or a save fails. Never alarming, always reassuring.
+export const ERRORS: Line[] = [
+  { zh: '哎呀～出了点小问题，你的字都还在哦。', en: 'Oops — a little hiccup, but your words are safe.' },
+  { zh: '哎呀，我这边卡了一下，马上就好。', en: 'Haiya, I got stuck for a sec — back in a moment.' },
+  { zh: '别担心，等一下再试试看 🍵', en: "Don't worry — let's try again in a bit. 🍵" },
+]
+
+// Word-count milestones worth a little cheer — every 100 words, on up.
+export const MILESTONES = Array.from({ length: 100 }, (_, i) => (i + 1) * 100)
 
 export function milestoneLine(n: number): Line {
   return {