[codex] Fix formatBytes edge cases

[Jaco-Ren]

Summary

• Clamp formatBytes to supported units through EB so very large values do not render with an undefined unit.
• Return a stable placeholder for negative, NaN, and infinite values instead of producing invalid output.
• Add focused Vitest coverage for byte formatting across normal, invalid, and out-of-range inputs.

Fixes #15.

Validation

• npm.cmd test -- --run src/lib/utils.test.ts
• npm.cmd run typecheck
• npm.cmd run build