perf(web): batch folder counts, bounded scroll scan, adaptive thumbs, lazy preview

Six-item frontend performance pass on the SvelteKit app.

P1 — Move per-folder photo counts to a new sidecar endpoint and defer
the fetch to requestIdleCallback. The old client-side path fired one
/photos?count=1000 per folder from the browser (≈1 MB JSON × N folders)
on every cold sidebar mount; the new POST /api/sidecar/folders/counts
fans out over loopback with bounded concurrency and returns a single
{path: count} payload of a few KB.

P2 — Bound the visibleRange scroll-scan around the previous visible
band instead of sweeping every shell from index 0 on each scroll-rAF.
Falls back to a full sweep on cache miss (filter reset, programmatic
jump) so behaviour is unchanged at the edges.

P3 — Adaptive thumbnail size + srcset. PhotoTile now picks the smallest
PhotoPrism tile_* variant (100/224/500) that covers the user's grid
preset at the current DPR. Adds decoding="async".

P4 — Lift the selection check above the {#each} loop. Mostly readability
— SvelteSet.has() is already per-key reactive — but keeps the hot loop
body terse.

P5 — Split dedupedAll / photos derivations so filter-store mutations
(search-as-you-type, section toggles) don't re-walk every loaded page;
only the cheap folder-scope filter re-runs.

P6 — Dynamic-import PreviewOverlay on first preview.uid !== null and
cache the loaded module; closing the overlay leaves the component
mounted with its internal {#if} collapsing the DOM.

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

web/src/routes/+page.svelte

@@ -143,16 +143,20 @@
 	/** Flattened view of every loaded page, deduplicated by UID. Adjacent
 	 *  pages can repeat a photo when its file-row span straddles the offset
 	 *  boundary (a `merged=true` quirk); the Set keeps first occurrence and
-	 *  preserves order. Downstream (`setOrder`, `rows`, preview, click
-	 *  handlers) treat this as the single source of truth.
+	 *  preserves order.
+	 *
+	 *  Split into two derived values so the heavy dedup pass only runs
+	 *  when the query's pages array changes (i.e. a fetch landed). The
+	 *  cheap folder-scope filter then runs whenever the user flips the
+	 *  filter store — search-as-you-type, section toggle, root vs.
+	 *  subfolder — without re-walking every page on each keystroke.
 	 *
 	 *  When the user picks the root entry in the folder tree we filter
 	 *  to `Path === ''` here — PhotoPrism's `path:` operator can't
 	 *  express that match, so the query fetches the whole library and
 	 *  we strip subfolder rows post-hoc. */
-	const photos = $derived<PpPhoto[]>(
-		applyFolderScope(dedupedPhotos(photosQuery.data?.pages), filters)
-	);
+	const dedupedAll = $derived<PpPhoto[]>(dedupedPhotos(photosQuery.data?.pages));
+	const photos = $derived<PpPhoto[]>(applyFolderScope(dedupedAll, filters));
 	function dedupedPhotos(pages: PpPhoto[][] | undefined): PpPhoto[] {
 		if (!pages) return [];
 		const seen = new Set<string>();
@@ -287,6 +291,15 @@
 		Math.max(visLast, forcedExpand ?? visLast) + TILE_BUFFER
 	);
 
+	// Hoist the selection reads out of the per-tile each-block. The same
+	// `has` / `===` semantics as `isSelected(uid) || selection.focused === uid`,
+	// just expressed at the loop level so the render reads as "given this
+	// (selected, focused) snapshot, here's each tile's state". No reactivity
+	// change — SvelteSet's `has` already subscribes per-key — but it removes
+	// the function-call indirection and keeps the hot loop body terse.
+	const selectedIds = $derived(selection.ids);
+	const focusedUid = $derived(selection.focused);
+
 	// Reset the window when the filter key changes — old indices from the
 	// previous photo list otherwise pin renderFirst/renderLast outside the
 	// new shorter list and the grid renders empty. Track `filtersToQ` as
@@ -878,7 +891,7 @@
 							use:tileRegister={i}
 						>
 							{#if inWindow}
-								{@const sel = isSelected(photo.UID) || selection.focused === photo.UID}
+								{@const sel = selectedIds.has(photo.UID) || focusedUid === photo.UID}
 								<PhotoTile
 									{photo}
 									selected={sel}