fix: Use faint color for disabled on-state toggle switch

[hanjinliu]

In the current implementation, we use the same toggle swtich groove color for enabled+off, disabled+on and disabled+off. This makes the check state unclear when the widget is disabled.

In this PR, on-color and off-color are mixed so it is visually easier to find the on-state switches, and they do not stand out too much.

Examples of enabled vs disabled toggle switches:

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.37%. Comparing base (cd67217) to head (345de6a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/superqt/switch/_toggle_switch.py	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #325      +/-   ##
==========================================
+ Coverage   86.35%   86.37%   +0.02%     
==========================================
  Files          49       49              
  Lines        3862     3868       +6     
==========================================
+ Hits         3335     3341       +6     
  Misses        527      527              

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[tlambert03]

Fine by me! Thanks

[tlambert03]

Fine by me! Thanks