fix(web): improve marketplace install dialog accessibility

[lyzno1]

Summary

• Expose the current installation step through DialogTitle so the dialog has an accessible name.
• Keep the close control in the focus loop while allowing the first step-owned safe action to receive initial focus.
• Cover the accessible name, initial focus, and failed-state title with focused tests.

Root cause

The visible heading was a plain div, so assistive technology could not derive the dialog name from it. The close button also appeared before the step content in DOM order, causing the focus manager to select it as the default initial focus target.

Validation

• pnpm --dir web test -- app/components/plugins/install-plugin/install-from-marketplace/tests/index.spec.tsx (64 tests passed)
• pnpm --dir web type-check
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.47%. Comparing base (6e5ba18) to head (d5a81f0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38798      +/-   ##
==========================================
- Coverage   85.47%   85.47%   -0.01%     
==========================================
  Files        5025     5025              
  Lines      273457   273457              
  Branches    51259    51259              
==========================================
- Hits       233729   233728       -1     
- Misses      35238    35239       +1     
  Partials     4490     4490              

Flag	Coverage Δ
dify-ui	94.61% <ø> (ø)
web	85.15% <ø> (-0.01%)	⬇️

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