bug/refactor: draw average CPU last, refactor CPU data code

[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:

This PR makes it so we draw the average CPU on top again. This also refactors internals to have the average CPU stored separately.

Issue

If applicable, what issue does this address?

#1802

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]

Codecov Report

❌ Patch coverage is 70.83333% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.30%. Comparing base (58a9a64) to head (c22b6b7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/canvas/widgets/cpu_graph.rs	50.00%	17 Missing ⚠️
src/app/data/time_series.rs	81.81%	2 Missing ⚠️
src/canvas/widgets/cpu_basic.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1804       +/-   ##
===========================================
+ Coverage   42.04%   53.30%   +11.26%     
===========================================
  Files         115      115               
  Lines       16174    16197       +23     
===========================================
+ Hits         6800     8634     +1834     
+ Misses       9374     7563     -1811     

Flag	Coverage Δ
macos-14	37.33% <1.38%> (-0.06%)	⬇️
ubuntu-latest	55.44% <70.83%> (+11.84%)	⬆️
windows-2022	37.71% <1.38%> (-0.07%)	⬇️

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