feature: split usage into usage percentage and value

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Denotes both usage and usage percentage. This also redoes the calculation for percentage to be based on the sum of avail + used, rather than on total, as otherwise we get potentially confusing percentages.

Issue

If applicable, what issue does this address?

Closes: #

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov-commenter]

Codecov Report

Base: 20.75% // Head: 20.69% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (0bb12ee) compared to base (0adefab).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   20.75%   20.69%   -0.07%     
==========================================
  Files          75       75              
  Lines       14577    14622      +45     
==========================================
  Hits         3026     3026              
- Misses      11551    11596      +45     

Impacted Files	Coverage Δ
src/app.rs	0.00% <0.00%> (ø)
src/data_conversion.rs	12.43% <0.00%> (-0.09%)	⬇️
src/widgets/disk_table.rs	0.00% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.