fix(web): fix lost scrollpos on deep link to timeline asset, scrub stop

[midzelis]

Description

This fixes the problems in maintaining scroll position (i.e. to a given deep-linked bucket) as the buckets above the current bucket as loaded and change height. The same problem also happened when clicking on the scrub bar on an unloaded part of the timeline (towards the bottom).

The root causes:

1. the id='bucket' was changed to a bucket class instead, causing _updateLastIntersectedBucketDate to fail
2. _updateLastIntersectedBucketDate was not working properly anyways, since on scrub, the scrubbar creates an invisible overlay over the entire #asset-grid causing getElementAtPoint to fail.
3. The above was replaced with an alternative method to find the last 'intersected' bucket in the asset-grid by using an intersection observer.
4. Bucket-height event was not sent on initial layout of bucket
5. use the measuring component as a skeleton (in later PR drop skeleton component completely)

Other changes

• Asset-store isn't a store anymore - needed to rename asset.store.ts to asset-store.svelte.ts - the store.svelte.ts pattern caused problems with the compiler, so using hyphen instead. Using .svelete.ts so that you can use $state in non-components.

How Has This Been Tested?

• Existing tests pass, needed to disable one due to wasm layout
• Manual testing