#169 made the filter drawer shared, which was the right first move and not the finish. Per-screen differences are booleans, the bar around the drawer was never shared at all, and the storefront's availability preset sits outside the system because the shared component cannot express "this belongs in the bar, not the drawer".
The design replaces the flags with composition: a screen contributes a list of filter dimensions, each declaring where it renders, how to render it, and what chips it contributes. A screen-specific control becomes an ordinary dimension, appearing in the chip row and counting toward the tally without the shared code knowing what it is.
Dimensions are plain data rather than components or context, for a concrete reason rather than a stylistic one. The drawer sets destroyOnHidden, so its sections are unmounted whenever it is closed — exactly when the chip row matters most. Anything that registers on mount would lose those chips the moment the drawer closed, which rules out the otherwise-idiomatic context-and-children approach.
The tally becomes the number of chips, so the count and the chip row cannot disagree — today they are computed by two routes and agree by coincidence, which the admin already has to correct by hand. Three visible behaviours change as a result, recorded in the spec rather than left to be discovered.
Adding vitest is in scope. The design's value rests on chips() being pure, and the frontend has no unit runner at all, so without one the core of it would ship covered only indirectly and expensively through Playwright.
The spec also records what is deliberately untouched: the filter state, the URL serialisation, the backend, and the two e2e assertions already failing on #186 — which must not be read as regressions from this work.
Refs #188
bermudalamb
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