feat: implement theme switching with system preference support

[AlliotTech]

• Show explicit theme state with system/sun/moon icons in the header
  toggle.
• Add a subtle transition animation when switching theme modes.
• Support “system” theme preference in the layout logic.

To maintain a consistent user experience, the dark theme is still used as the default.

related issue: #851

Preview:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.15%. Comparing base (bf16f1c) to head (ff4418d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #902   +/-   ##
=======================================
  Coverage   79.15%   79.15%           
=======================================
  Files          56       56           
  Lines        2226     2226           
=======================================
  Hits         1762     1762           
  Misses        360      360           
  Partials      104      104           

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[eternal-flame-AD]

Thanks for the PR, see comments please