refactor(web): extract asset grid layout logic into AssetLayout component

- Extracts the asset grid rendering logic from `MonthSegment` into a
  dedicated `AssetLayout` component
- Simplifies `MonthSegment` by delegating layout responsibilities
  while maintaining all existing functionality
- Renames `customLayout` prop to `customThumbnailLayout` for clarity
  across Timeline components


## Changes
  - Created new `AssetLayout.svelte` component that handles:
    - Asset grid rendering with proper positioning
    - Animation transitions
    - Filtering of intersecting viewer assets
  - Updated `MonthSegment.svelte` to use `AssetLayout` via composition
  pattern
  - Renamed `customLayout` to `customThumbnailLayout` in Timeline and
  related components
  - Moved thumbnail click and selection logic to Timeline parent
  component using snippets

web/src/lib/managers/photostream-manager/PhotostreamSegment.svelte.ts

@@ -152,12 +152,15 @@ export abstract class PhotostreamSegment {
     if (ids.size === 0) {
       return {
         moveAssets: [] as MoveAsset[],
+        // eslint-disable-next-line svelte/prefer-svelte-reactivity
         processedIds: new Set<string>(),
         unprocessedIds: ids,
         changedGeometry: false,
       };
     }
+    // eslint-disable-next-line svelte/prefer-svelte-reactivity
     const unprocessedIds = new Set<string>(ids);
+    // eslint-disable-next-line svelte/prefer-svelte-reactivity
     const processedIds = new Set<string>();
     const moveAssets: MoveAsset[] = [];
     let changedGeometry = false;