fix(web): fill dataset create layout height

[lyzno1]

Summary

• let the dataset create page inherit the main layout height instead of subtracting the nav height again
• replace the inner percentage height calculation with flex growth so the panel background reaches the bottom

Verification

• pnpm -C web exec eslint app/components/datasets/create/index.tsx (passes with existing react/use-state warnings)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.29%. Comparing base (451faa0) to head (d8a1703).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #38308   +/-   ##
=======================================
  Coverage   85.29%   85.29%           
=======================================
  Files        4982     4982           
  Lines      263228   263228           
  Branches    50023    50023           
=======================================
  Hits       224509   224509           
  Misses      34335    34335           
  Partials     4384     4384           

Flag	Coverage Δ
dify-ui	94.93% <ø> (ø)
web	84.86% <ø> (ø)

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