Thousands separator for byte counts in human_size() (+ first test)

[Vortrix5]

Summary

human_size() formats most sizes with a thousands separator (1,023.5 MB), but the bytes branch doesn't: 5000 shows as 5000 B instead of 5,000 B. Small consistency fix, and human_size has no test yet, so this is a nice spot to add the first one.

Where

src/sifty/console.py, the human_size() function. The B case uses f"{int(value)} B".

Acceptance criteria

• Byte counts use a thousands separator (e.g. 5,000 B)
• A test covering a few sizes across units (B / KB / MB)
• pytest green

Tip

f"{int(value):,} B" does the formatting. Add the test to tests/ (a new test_console.py is fine).