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

[giuseppelanzi]

Closes #44

What

Adds 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 and a total on Clear All.

Why

The tool only shows total free space and a before/after figure. There was no
way to see which options are worth running, or how much each would reclaim,
before deleting anything.

How

• Strictly read-only: only runs du (bash) / Measure-Object
  (PowerShell). Deletes nothing; behaves identically under --dry-run.
• Per-entry estimate mirrors exactly the paths the matching cleanup routine
  would remove (kept in a dedicated, commented section).
• Best-effort for tool-managed caches (npm/yarn/pnpm, Homebrew, …) via their
  known cache dirs; approximate values are prefixed with ~.
• Flutter and PlatformIO cover the global cache only (fast, deterministic,
  no recursive project scan); Time Machine reports the local snapshot count.
• macOS sizes use the same binary units as df -h (Gi/Mi/Ki) so the
  estimate matches the existing "Free Space" figure.
• Implemented in both dev-cleaner.sh and dev-cleaner.ps1.
• Option 99 is intentionally non-contiguous and categories use stable
  string keys, so existing options 0-N, the "Invalid choice" message and
  any future menu renumbering are unaffected.

Testing

• bash -n dev-cleaner.sh: passes.
• shellcheck dev-cleaner.sh: no new findings introduced by this change
  (only pre-existing, unrelated warnings remain; the two intentional
  glob-expansion lines mirror safe_rm and carry a scoped
  # shellcheck disable with a reason).
• Unit tests of the helpers (human_kb, du_kb_sum, est) and an
  end-to-end estimate_all run against a sandboxed $HOME: all 14
  categories + total populated, approximations marked ~, Time Machine as
  a count, and a before/after filesystem snapshot confirming nothing is
  deleted (read-only guarantee, identical under --dry-run).
• dev-cleaner.ps1: feature implemented at parity (no PowerShell available
  on the dev machine to execute it locally — Windows validation by reviewers
  welcome).

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