[EW] Validate image size limits client-side before upload in EW

Currently, oversized images are not validated until publish time, causing a publish error with no early warning. The image size limit should be checked client-side on drop/upload so that users receive immediate feedback instead of discovering the issue only at publish.

Background: Helix has defined image size limits. These limits can be exposed via the Sidekick config in the Admin API, making them accessible to EW. (XWalk already has this check as well, DA does not)

Source: https://jira.corp.adobe.com/browse/SITES-46294

Start by tracing EW’s image drop/upload handling and how Sidekick configuration from the Admin API is consumed. Verify how Helix image-size limits are represented and compare the existing XWalk check. Done when oversized images receive immediate feedback on drop/upload instead of being discovered only at publish time.

Written by the indexing model from the issue text.