Consolidate format_size() into src/utils.rs

**TaskGuard ID:** backend-025  
**Task File:** [tasks/backend/backend-025.md](https://github.com/Guard8-ai/TaskGuard/blob/master/tasks/backend/backend-025.md)
**Source Branch:** master
**Hash:** 8e0d4ae7

## Description

[Why this task exists and what problem it solves]

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*Synced from TaskGuard*