feat(dashboard): refine section skeleton loaders

[nobledeveloper01]

Per-section dashboard skeletons already landed in #7, so this PR closes the gap that was left behind and modernises the loading state. Closes #17.

Fixes a real layout shift: StatCardSkeleton was never updated when the stat cards gained their coloured icon chips, leaving it ~20px shorter than the loaded card so everything below it jumped when data arrived. The skeleton now mirrors the card exactly, measured at 0px shift (134px skeleton, 134px loaded).

Also replaces the opacity pulse with a sweeping shimmer, gives the charts a silhouette (plotted bar series with axes, and a donut ring) instead of flat grey blocks, staggers the sweep across the stat cards, and adds a slight upward drift to the fade so content settles rather than pops.

Reduced motion is respected: the sweep is hidden and the static tint remains. The shimmer lives in the shared Skeleton primitive, so the recommendations and accounts pages inherit it.

Stacks on #26 and #27; those commits drop off as they merge.

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