docs: prefer shadcn/ui for frontend components

[Roy-Kid]

Re-lands the only live content from the stale #6 (which was 207 commits behind dev; its CI/pre-commit commit is already on dev). Documents the shadcn/ui component preference in CLAUDE.md. Closes #5.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.09%. Comparing base (97dddd2) to head (d69532e).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev       #9   +/-   ##
=======================================
  Coverage   79.09%   79.09%           
=======================================
  Files         264      264           
  Lines       16374    16374           
=======================================
  Hits        12951    12951           
  Misses       3423     3423           

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