Display table size in GB if above 999 MB, not always MB

Issue:
Currently, the "Tables size" and "Cache tables size" sections in the dashboard always show table sizes in MB in the UI, even if a table exceeds 1,000 MB, which makes large tables difficult to read at a glance.

Proposed improvement:
Display table sizes in GB (with two decimals) if the value is greater than 999 MB, and in MB otherwise. This would make the reporting more intuitive and improve the end-user experience for large databases.

Suggested solution:
Format the table size strings in the controller before passing to Twig.
Remove the hardcoded MB from the Twig file and let the data include the units ("MB" or "GB") as appropriate.

Benefits:
Makes table sizes for large tables much more readable.
Keeps template logic minimal and future-proof.