Show human-readable file size units in the max file size validation message

Problem/Motivation

Issue #3590178 fixed the mismatched Drupal.t() placeholder in the client-side max file size validation (s3fs_cors.js, max_size()), so the error no longer renders raw <em> markup. The message still only reports the limit in MB though, so a 5 GB limit reads "Max size allowed is: 5120 MB", which is hard to read.

Proposed resolution

Format the byte limit into the largest sensible unit (bytes/KB/MB/GB/TB) so the message reads "Max size allowed is: 5 GB" (or "10 MB", and so on), consistent with how core's ByteSizeMarkup presents sizes. See the attached merge request.

Remaining tasks

Review and merge the MR.

User interface changes

The max file size validation error now shows human-readable units (for example 5 GB) instead of a raw MB number.

API changes

None.

Data model changes

None.