ci: ensure pnpm is available in setup-web action

[lyzno1]

Summary

• add setup-web action metadata description
• install pnpm with the official pnpm/action-setup action before Vite+ setup
• let pnpm/action-setup read the repository packageManager field and avoid a duplicate install

close #36317

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/actions/setup-web/action.yml"); puts "yaml ok"'
• go run github.com/rhysd/actionlint/cmd/actionlint@latest -ignore 'label ".*" is unknown'
• git diff --check
• pnpm --version

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.93%. Comparing base (28818f2) to head (5d7582d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #36315   +/-   ##
=======================================
  Coverage   85.93%   85.93%           
=======================================
  Files        4456     4456           
  Lines      211260   211260           
  Branches    39487    39487           
=======================================
  Hits       181556   181556           
  Misses      26500    26500           
  Partials     3204     3204           

Flag	Coverage Δ
dify-ui	94.59% <ø> (ø)
web	86.55% <ø> (ø)

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