Merge feat/translate: canTranslate + Opus-MT transcript translation (spec 001 workstream 4)

Co-Authored-By: Claude Opus 5.5 <[email protected]>

# Conflicts:
#	docs/Decisions.md

docs/WorkBreakdown.md

@@ -65,6 +65,8 @@ Notes (M1):
       surface active backend in UI
 - [x] Handle model switching (dispose old pipeline, load new)
 - [x] Language / task options for multilingual models (transcribe vs translate)
+- [x] Whisper translate only on models trained for it (`canTranslate`; hidden for
+      large-v3-turbo and `.en`, stale "translate" resets to transcribe — D-WS4a)
 - [~] Verify quantized variants (q4/q8/fp16) on both backends; record working
       combinations in `models.ts`
 
@@ -347,6 +349,16 @@ cached.
 
 ---
 
+## Spec 001 — Translate (workstream 4)
+- [x] `canTranslate` per model; Task control hidden where false; stale
+      "translate" resets to transcribe (D-WS4a)
+- [x] Text translation of any finished transcript, da→en / en→da, Opus-MT
+      in its own worker, sentence-chunked, timestamps kept for exports (D-WS4b)
+- [~] Real-browser checks: offline translation (`tests/offline.md` §5),
+      WebGPU-machine speed (owner)
+
+---
+
 ## Deliberately out of scope (for now)
 Arbitrary Hub model ids, OAuth providers, multi-user transcript storage,
 whisper.cpp server backend, i18n of the UI.
@@ -378,6 +390,14 @@ Found while bootstrapping agent docs (D-036); all fixed on 2026-10-29.
       — Light theme via tokens + self-hosted Inter (D-037). Also fixed: the
       Backend select overflowed its panel on 390 px-wide phones.
 
+Found while building transcript translation (D-WS4b), 2026-10-29:
+- [ ] Transformers.js 4.2 batched Marian generation returns row 0's output for
+      every row of a padded batch, so translation runs one sentence per call
+      (~4× slower than batch 8 would be). Re-check after the 4.3 upgrade with
+      `bun scripts/bench-translate.ts` ("output identical" across batch sizes).
+- [ ] The server API still accepts `task: "translate"` with large-v3-turbo
+      (only the UI hides it, D-WS4a).
+
 ## Recurring
 - [ ] Maintain `AGENTS.md`:
   - **Add a rule only in response to a real failure.** When an agent repeatedly