refactor(web): switch consumers to use day-tier viewport boundaries

TimelineDay.isInOrNearViewport now derives from the firstInOrNearIndex
$state added in the previous commit (true iff first index != -1). This
replaces the old $derived.by that read every asset's isInOrNearViewport
via viewerAssets.some(), removing a per-asset subscription point that
filter() in AssetLayout had been creating for every render.

AssetLayout switches from filterIsInOrNearViewport(viewerAssets) to
viewerAssets.slice(firstInOrNearIndex, lastInOrNearIndex + 1). The slice
expression depends only on the two boundary $state values, not on any
asset's proximity $derived. Reactive churn during scroll collapses to:
boundary indices change → slice recomputes → {#each} reconciles.

Month.svelte passes the new boundary props through. filterIsInOrNearViewport
is still used at the month tier (to filter days) and stays in utils.

Change-Id: If4e30192146f3e987307b1efd7c6d41d6a6a6964

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

@@ -1,6 +1,5 @@
 <script lang="ts">
   import type { TimelineAsset } from '$lib/managers/timeline-manager/types';
-  import { filterIsInOrNearViewport } from '$lib/managers/timeline-manager/utils.svelte';
   import type { ViewerAsset } from '$lib/managers/timeline-manager/viewer-asset.svelte';
   import type { VirtualScrollManager } from '$lib/managers/VirtualScrollManager/VirtualScrollManager.svelte';
   import { uploadAssetsStore } from '$lib/stores/upload';
@@ -13,6 +12,8 @@
 
   type Props = {
     viewerAssets: ViewerAsset[];
+    firstInOrNearIndex: number;
+    lastInOrNearIndex: number;
     width: number;
     height: number;
     manager: VirtualScrollManager;
@@ -27,15 +28,27 @@
     customThumbnailLayout?: Snippet<[asset: TimelineAsset]>;
   };
 
-  const { viewerAssets, width, height, manager, thumbnail, customThumbnailLayout }: Props = $props();
+  const {
+    viewerAssets,
+    firstInOrNearIndex,
+    lastInOrNearIndex,
+    width,
+    height,
+    manager,
+    thumbnail,
+    customThumbnailLayout,
+  }: Props = $props();
 
   const transitionDuration = $derived(manager.suspendTransitions && !$isUploading ? 0 : 150);
   const scaleDuration = $derived(transitionDuration === 0 ? 0 : transitionDuration + 100);
+  const visibleViewerAssets = $derived(
+    firstInOrNearIndex === -1 ? [] : viewerAssets.slice(firstInOrNearIndex, lastInOrNearIndex + 1),
+  );
 </script>
 
 <!-- Image grid -->
 <div data-image-grid class="relative overflow-clip" style:height={height + 'px'} style:width={width + 'px'}>
-  {#each filterIsInOrNearViewport(viewerAssets) as viewerAsset (viewerAsset.id)}
+  {#each visibleViewerAssets as viewerAsset (viewerAsset.id)}
     {@const position = viewerAsset.position!}
     {@const asset = viewerAsset.asset!}
 

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

@@ -101,6 +101,8 @@
     <AssetLayout
       {manager}
       viewerAssets={timelineDay.viewerAssets}
+      firstInOrNearIndex={timelineDay.firstInOrNearIndex}
+      lastInOrNearIndex={timelineDay.lastInOrNearIndex}
       height={timelineDay.height}
       width={timelineDay.width}
       {customThumbnailLayout}

web/src/lib/managers/timeline-manager/timeline-day.svelte.ts

@@ -22,12 +22,12 @@ export class TimelineDay {
 
   height = $state(0);
   width = $state(0);
-  isInOrNearViewport = $derived.by(() => this.viewerAssets.some((viewAsset) => viewAsset.isInOrNearViewport));
 
   // Indices into viewerAssets bounding the in-or-near range. -1/-1 means no assets are in-or-near.
   // Updated imperatively by updateAssetBoundaries() from updateViewportProximities() and layout().
   firstInOrNearIndex = $state(-1);
   lastInOrNearIndex = $state(-1);
+  isInOrNearViewport = $derived(this.firstInOrNearIndex !== -1);
 
   #top: number = $state(0);
   #start: number = $state(0);