fix(web): add backdrop blur to skip nav

[lyzno1]

Summary

• Add backdrop blur to the skip navigation link so the translucent secondary-button surface remains readable in dark mode.

Tests

• pnpm -C web test app/components/main-nav/tests/layout.spec.tsx app/components/main-nav/tests/skip-nav.spec.tsx

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (2c6ec1a) to head (065a04e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #38517   +/-   ##
=======================================
  Coverage   85.33%   85.33%           
=======================================
  Files        5005     5005           
  Lines      265421   265421           
  Branches    50474    50474           
=======================================
  Hits       226504   226504           
  Misses      34469    34469           
  Partials     4448     4448           

Flag	Coverage Δ
dify-ui	94.72% <ø> (ø)
web	84.86% <ø> (ø)

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