fix(ui): use proper decimal prefixes (kB / MB) in file-size selector

[filipchristiansen]

What it does

• Updates the file-size slider label in git_form.jinja and the formatSize helper in utils.js to show kB / MB instead of the misleading kb / mb.
• No behaviour or API changes.

Why

• Lower-case “b” denotes bits. Our UI currently claims kilobits/megabits while the backend logic actually works in bytes. This patch fixes that mismatch.

Discussion

• We’re standardising on correct decimal prefixes for now. I’m still weighing a full switch to IEC binary prefixes (KiB/MiB) and have asked @cyclotruc for input.

Thanks

• Shout-out to @Self-Perfection for catching the issue in Use proper KiB and MiB instead of kilo-/mega- bits #293