fix(web): update snippet placeholder icon color

[JzoNgKVO]

Summary

update snippet placeholder icon color

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.27%. Comparing base (94c0967) to head (ff620d4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #38445   +/-   ##
=======================================
  Coverage   85.27%   85.27%           
=======================================
  Files        5004     5004           
  Lines      264911   264911           
  Branches    50366    50366           
=======================================
  Hits       225904   225904           
  Misses      34569    34569           
  Partials     4438     4438           

Flag	Coverage Δ
dify-ui	94.77% <ø> (ø)
web	84.85% <ø> (ø)

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