Align GLQL table shadow with sticky header tables

What does this MR do and why?

Implements the follow-up Nick suggested in #585265 (comment 3218560057): keep the GLQL table scroll shadow consistent with the new sticky header table treatment: we're showing the shadows only to the right (where the overflowing content is) until we reached the end. There is no shadow to the left anymore.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create an issue with a GLQL table wide enough to overflow, for example:

   ```glql
   display: table
   fields: title, state, author, created, updated, milestone, weight, labels, assignee
   limit: 5
   query: project = "flightjs/Flight"
   ```

2. View the issue and scroll the table horizontally

3. Verify: a shadow on the right edge while more content exists to the right, no shadow on the left at any scroll position, shadow fades out when reaching the end