Better error handling

docs/WorkBreakdown.md

@@ -218,8 +218,14 @@ Notes (M5):
   exercised for real (browser half still needs the manual run).
 
 ## M6 — Hardening & polish
-- [ ] Error taxonomy: unsupported file, out-of-memory (suggest smaller model),
-      WebGPU device-lost, network loss mid-download (resume or clear retry)
+- [x] Error taxonomy: unsupported file, out-of-memory (suggest smaller model),
+      WebGPU device-lost, network loss mid-download (resume or clear retry) —
+      pure `classifyError` in `src/lib/errors.ts` (D-030) sorts every failure
+      path (decode, load, transcribe, a crashed worker) into one of the four
+      categories plus a generic fallback, each with a user-facing message and
+      an actionable suggestion; the error box in `App.tsx` adds a Retry
+      button for the two retryable kinds (network, device-lost), replaying
+      the failed load or the last file. Unit-tested in `tests/errors.test.ts`.
 - [x] Cancel in-flight transcription (both engines) — browser engine via
       worker termination (D-018), server engine via aborting the
       `/api/transcribe` fetch (D-018 amendment; landed with M5's EngineToggle)