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/lib/services/photoprism.ts

@@ -364,33 +364,32 @@ export async function getImportInfo(): Promise<ImportInfo> {
 
 /**
  * Per-folder photo count for each `paths[]` entry. PhotoPrism's `/folders`
- * endpoint reports `FileCount: 0` even when populated, and the `/photos`
- * response has no total-rows header — X-Count is the per-page row count.
- * So we fire one `/photos?q=path:X&count=1000` per folder and dedupe by
- * UID — `merged=false` returns one row per FILE, so a HEIC+JPG companion
- * pair counts twice if we trusted `data.length`. Capped at the server's
- * 1000-row ceiling; folders that overflow render as "1000+" in the UI.
+ * endpoint reports `FileCount: 0` even when populated, so the count has
+ * to be derived from a `/photos?q=path:X` lookup per folder.
+ *
+ * We hand this off to the sidecar (`POST /api/sidecar/folders/counts`)
+ * which fans out to PhotoPrism over loopback, dedupes by UID, and
+ * returns a single `{path: count}` payload of <5 KB. The previous
+ * client-side implementation issued one `/photos?count=1000` per folder
+ * from the browser — on a library with 30 folders that's ≈30 MB of JSON
+ * pulled across the wire on every cold sidebar mount.
  *
  * `path:X` is non-recursive in PhotoPrism's q-DSL: it matches direct
- * children only, so summing the per-path counts (no double-counting from
- * nested folders) is the right way to derive the root-folder photo
- * count.
+ * children only, so summing the per-path counts (no double-counting
+ * from nested folders) is the right way to derive the root-folder
+ * photo count. Capped at PhotoPrism's 1000-row ceiling; folders larger
+ * than that under-report (pre-existing limitation, unchanged here).
  *
- * Returns a plain object keyed by the input paths to keep it JSON-friendly
- * for TanStack's structural sharing.
+ * Returns a plain object keyed by the input paths to keep it JSON-
+ * friendly for TanStack's structural sharing.
  */
 export async function listFolderCounts(paths: string[]): Promise<Record<string, number>> {
-	const entries = await Promise.all(
-		paths.map(async (path) => {
-			const { data } = await http.get<PpPhoto[]>('/photos', {
-				params: { count: 1000, offset: 0, merged: false, q: `path:${path}` }
-			});
-			const uids = new Set<string>();
-			for (const p of data) uids.add(p.UID);
-			return [path, uids.size] as const;
-		})
-	);
-	return Object.fromEntries(entries);
+	if (paths.length === 0) return {};
+	const data = (await sidecar('POST', '/folders/counts', { paths })) as Record<
+		string,
+		number
+	>;
+	return data;
 }
 
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