feat(workspaces): content-driven canvas while dragging — scrollbar grows/shrinks with the gesture

Revert the fixed append-canvas reservation (9ede1f0): pre-reserving the 3
spaces at drag start made the view jump right and created far too much free
canvas. The grid is back to hugging its content (width: max-content / auto),
so the preview extends the canvas as the tile reaches toward the edge and it
shrinks back when the gesture ends — the scrollbar moves with the drag, as
intended. The drag-target clamp (dragAppendLimit, +3 past the committed
edge) still bounds how far the preview can extend, so the "endless growth"
problem stays solved without reserving anything up front.

Dots restore the one-pitch padding extension for the append slot; the
scroll-measure effect still re-runs on drag start/end (the canvas shrinks
back on cancel). Test reverted to the content-driven assertions.

Verification: svelte-check 0 errors/0 warnings; vitest 1693/1693; biome
clean on touched files; build ok.

0 files changed, 0 insertions, 0 deletions