feat(workspaces): reserve the append canvas for the whole drag — no scrollbar change mid-drag

Previously the grid kept `width: max-content` during a drag, so the canvas
grew/shrunk as the preview placed the tile at each hovered cell — the
scrollbar resized mid-drag, making placement harder. Now the moment a drag
starts the grid's growth axis becomes a FIXED reservation: the append limit
(committed content edge + 3, span-aware) computed at drag start, so the 3
spaces "pop in" immediately, the scroller size is constant for the entire
gesture, and the unused reservation disappears when the drag ends (the
canvas returns to max-content / auto). The dragged tile's span room is
included so its body always stays inside the reserved canvas.

Also: the scroll-measure effect now tracks `dragging`, so canPan and the
scroll indicator re-measure when the reservation appears and disappears.

Test: the drag test now asserts the grid's style is a fixed px width while
dragging (never max-content) and back to max-content after the drop. (This
caught a missing `px` unit that would have silently voided the reservation.)
Update the handoff bullet to describe the reservation.

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

0 files changed, 0 insertions, 0 deletions