refactor(structure): migrate remaining features and lock the convention

## Problem
Remaining ~60 files across study/tasks/data-viz/devices/profile still live in layer folders after the pilot.

## Proposed fix
One PR per feature, lowest-risk first (data_viz → devices → tasks → study/home last, since it depends on the app coordinator from #604). Add a CI grep/custom-lint guard for the dependency rule.

## Acceptance criteria
- [ ] `lib/ui`, `lib/view_models`, `lib/blocs` folders are gone or empty
- [ ] CI guard for the dependency rule is active

**Effort:** L · **Priority:** P2 · **Change risk:** Low per-PR (mechanical moves after the cleanups)