Feature: estimate reclaimable space before cleaning (menu option 99)

[giuseppelanzi]

Summary

Add a read-only menu option 99 that estimates how much disk space
each cleanup entry would free, then redraws the menu with
(Estimate: <size>) next to every option plus a total on Clear All.

Today the tool only shows total free disk space and a before/after figure;
there's no way to know which options are worth running, or how much each
would reclaim, before deleting anything.

Proposed behaviour

• New option 99 (non-contiguous on purpose, so it stays stable if the menu
  is renumbered by future PRs). Existing options 0-N and the "Invalid
  choice" message are unchanged.
• Strictly read-only: it only runs du (bash) / Measure-Object
  (PowerShell). It deletes nothing and behaves identically under --dry-run.
• Per-entry estimate mirrors exactly the paths the matching cleanup routine
  would remove.
• Best-effort for tool-managed caches (npm/yarn/pnpm, Homebrew, etc.) using
  their known cache dirs; approximate values are marked with a leading ~.
• Flutter and PlatformIO estimates cover the global cache only (fast and
  deterministic, no recursive project scan); Time Machine reports the local
  snapshot count rather than bytes.
• Sizes use the same binary units as df -h on macOS (Gi/Mi/Ki) so the
  estimate matches the existing "Free Space" figure.
• Implemented in both dev-cleaner.sh and dev-cleaner.ps1 (parity).

Environment

• OS: macOS (bash 3.2-safe, validated with shellcheck) and Windows
  (PowerShell 5.1+) — feature added to both ports.
• Shell: bash / PowerShell.

I have an implementation ready and would like to submit it as a PR if the
maintainers are happy with the approach (especially: option number 99,
read-only guarantee, and the ~ = approximate convention).