fix: handle formatBytes edge cases

[Jaco-Ren]

Summary

Fixes formatBytes edge cases reported in #15:

• return an explicit placeholder for negative or non-finite byte values
• keep fractional byte values in the B unit instead of producing undefined
• add PB and EB units and clamp the selected unit index for very large values
• add a focused Vitest regression test for zero, byte, PB/EB, invalid, fractional, and beyond-EB inputs
• add the missing Vitest setup file referenced by vite.config.ts

This differs from the existing #13 by covering the >= 1 PiB case and preserving the invalid-size placeholder behavior requested in #15.

Verification

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

I also checked the touched files with ESLint. The full existing lint command still reports pre-existing no-explicit-any errors in src/components/code/BlueprintGraph.tsx, unrelated to this change.

Fixes #15