Better model download management

docs/WorkBreakdown.md

@@ -242,6 +242,11 @@ Notes (M5):
       models, versioned/cleaned by the SW itself) is deliberately not
       surfaced — deleting it per-model makes no sense and the SW would
       just re-fetch it.
+- [x] Model downloads are user-initiated (D-020): selecting a model/backend no
+      longer auto-downloads weights; the load starts on browser-engine
+      transcription start (file drop / live start) or via an explicit
+      per-model Download button in `ModelPicker` — and never when the engine
+      toggle is set to "server".
 - [ ] Basic a11y pass (keyboard, focus, aria on progress/live transcript)
 - [ ] Production build + `bun start` deployment check; README for deployers