fix: eliminate PieceList flash on pagination

[shaoster]

Problem

Fixes #734. On the PieceList page with 17+ pieces, a dark flash appears immediately after the first 16 cards render. Two bugs compound to cause it.

Fix

Bug 1 — Sentinel fires before masonry renders (PieceList.tsx):

The scroll sentinel useEffect called check() immediately when hasMore became true. At that moment masonryWidth=0 (the ResizeObserver hasn't fired yet), so the sentinel sits at top≈0 and onLoadMore fires before any cards are visible — triggering the second page fetch while the masonry is still empty.

Fix: add masonryWidth to the effect's dependency list and return early when masonryWidth === 0. The sentinel check is deferred until the masonry has rendered its cards and the sentinel is at its true document position.

Bug 2 — Full positioner reset on every append (PieceList.tsx):

The positioner was computed via useMemo with filteredPieces as a dependency. Every time the second page arrived and filteredPieces grew 16→32, createPositioner was called from scratch — discarding all cached positions — and MasonryScroller re-laid out all items from zero. That re-layout is the flash.

Fix: replace createPositioner + useMemo with masonic's usePositioner hook. usePositioner only resets when its deps array (sortOrder, active filters, active tags) or layout options (masonryWidth, columnWidth, isMobile) change — not when items are appended. Crop heights are seeded inline with a positioner.get(index) === undefined guard so only new unpositioned items are seeded; existing positions survive pagination.

Also removes the now-unused getMasonryColumns helper (its logic is handled internally by usePositioner).

Regression Tests

Two tests added to web/src/components/__tests__/PieceList.test.tsx:

• scroll sentinel > does not call onLoadMore before the masonry container has a measured width — renders with masonryWidth=0 and hasMore=true; asserts onLoadMore is never called. Failed before the fix (Bug 1).

• masonry height pre-seeding > does not re-seed existing crop heights when pieces are appended via pagination — renders with a cropped piece, appends a second piece, asserts positioner.set is still called exactly once (index 0 not re-seeded). Failed before the fix (Bug 2).

The beforeEach mock for mockPositioner.get was updated to be stateful (returns the value written by set), matching the real positioner's behaviour and enabling the "does not reseed on unrelated rerender" test to remain valid.

Verification

rtk bazel test //web:web_piece_list_test --test_output=all  # 53 tests pass
rtk bazel test //web:web_test --test_output=errors          # 37 test targets pass

Closes #734