docs: improve documentation and add theme toggle

[reugn]

• Add light/dark mode toggle with preference persistence
• Use light color scheme as default
• Pin gem versions for consistent builds
• Silence Sass deprecation warnings
• Fix timeout documentation accuracy

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.94%. Comparing base (3803e2b) to head (1c9995a).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files          18       18           
  Lines         968      968           
=======================================
  Hits          890      890           
  Misses         45       45           
  Partials       33       33           

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