#7 test: add unit tests for the format helpers

[Anonymous]

Originally created by: Caqil

lib/core/ui/app_format.dart has helpers like formatBytes(...) with no dedicated tests.

Add test/core/app_format_test.dart covering:

• bytes → "KB"/"MB"/"GB" boundaries
• 0 bytes, small values, rounding

Small, self-contained, and a great way to learn the test setup.