preview: LQIP + ±2 prefetch + bound action bar to its column

Preview pane paints instantly: a blurred copy of the same thumbnail the
grid loaded (cache hit) rides beneath the sharp fit_1280, which now
carries fetchpriority=high and decoding=async. A $effect prefetches
fit_1280 for the ±2 neighbours so arrow-skim hits the HTTP cache.
Carousel thumbs drop to fetchpriority=low so they yield to the main
image. Skeleton grid gains an mt-2 to breathe against the toolbar.
BulkActionBar moves inside the main column in both PreviewModal and the
/tags drill-in so it no longer stretches under the right sidebar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/timeline/SkeletonGrid.svelte

@@ -22,7 +22,7 @@
 
 <div
 	aria-hidden="true"
-	class="grid gap-2"
+	class="mt-2 grid gap-2"
 	style="grid-template-columns: {tracks};"
 >
 	{#each Array.from({ length: count }) as _, i (i)}