Correct support of SI/IEC units for filesize

[Altahrim]

Summary

Implements correct handling for SI/IEC units.
Add correct display for: #59965

TODO

• Quota frontend display SI units but actually use 1024 base (it sends bytes so it's not an issue for backend)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[artonge]

For the frontend part, maybe that's a design decision, cc @marcoambrosini or @kra-mo.

Also cc @susnux as you worked on it IIRC.

[susnux]

For the frontend part

would need to be adjusted in the files library. But I think this is a great opportunity to finally localize it.
E.g. 1000.00 MB would be 1,000.00 MB in English, but 1.000,00 MB in German and 1 000,00 Mo in French.

see also #51623