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/routes/tags/[category]/[[value]]/+page.svelte

@@ -226,6 +226,7 @@
 					<PhotoGrid photos={drillPhotos} />
 				{/if}
 			</main>
+			<BulkActionBar />
 		</div>
 
 		{#if !view.rightSidebarCollapsed}
@@ -270,5 +271,3 @@
 		{/if}
 	</div>
 {/if}
-
-<BulkActionBar />