refactor(dashboard): drop the vestigial DashboardApp.prefs field left by #586

Surfaced while shipping #586 (phase 1 of #593). Deferred rather than folded in, because
removing it is a DashboardApp-shape change with no bearing on #586's acceptance criteria.

What

src/ui/dashboard.ts:240 declares:

prefs: Pick<AppPreferences, 'save'>;

on the DashboardApp-shaped interface. Its only real justification was the cell-detail
drawer's resize persistence: openCellDetail → attachDrawerResize used to read
state.cellDrawerPx + prefs.save.

After #586 that path is gone. Cell detail from Dashboard always docks into the shell-owned
inspectorHost, and resize is owned by app-shell.ts writing the single rightInspectorPx
preference (one canonical write, src/ui/splitters.ts:140). So prefs.save is no longer
exercised through that call.

#586 updated the field's doc comment in place to say exactly this and deliberately kept the
field, so a future narrower caller gets a compile-time signal instead of a runtime crash. That
was the right call for a refactor phase whose contract said nothing about it — but the field is
now vestigial, and a comment explaining why a field is unused is a weaker guarantee than not
having the field.

Why deferred

Dropping it changes the DashboardApp structural shape and touches every construction site
plus the as ResultsApp cast next to it. That is decomposition work, not shell-primitive work.

Suggested home

Phase 5 of #593 (#589 — extract the dashboard tile gesture controller and a pure repaint
plan) already rewrites this file's seams and is the natural place to drop it. Phase 4 (#588,
composition-root decomposition) is the alternative if the field turns out to be load-bearing for
staged construction.

Not urgent, no user-visible effect, no correctness risk today.

Acceptance

• prefs is removed from src/ui/dashboard.ts's DashboardApp interface, or a comment
  records a current reason it must stay that does not rest on the deleted
  attachDrawerResize path.
• npm test, npm run check:types, npm run check:arch pass.

Start at src/ui/dashboard.ts:240 and trace every DashboardApp construction site, including the adjacent as ResultsApp cast. Check whether staged construction or the seams targeted by #589 and #588 still require prefs; then run npm test, npm run check:types, and npm run check:arch to confirm the field is removed or has a current justification.