fix(web): prevent plugin cards from overlapping marketplace panel

[lyzno1]

Summary

• remove the stale z-index from installed plugin card inner surfaces so they stay in normal scroll content paint order
• remove the same z-index from the matching plugin list skeleton
• drop the undefined hover-bg marker from plugin item and install loading surfaces; keep the documented page-feedback marker in marketplace cards untouched

Verification

• pnpm --dir web exec vitest run app/components/plugins/plugin-item/tests/index.spec.tsx
• git diff --check

Notes

• pnpm --dir web exec eslint app/components/plugins/plugin-item/index.tsx still reports existing any, non-interactive click handler, and Remix icon warnings/errors unrelated to this patch.
  Refs SNP-516

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.30%. Comparing base (1b3e8e9) to head (af34377).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38427      +/-   ##
==========================================
- Coverage   85.35%   85.30%   -0.06%     
==========================================
  Files        5159     5004     -155     
  Lines      269252   264921    -4331     
  Branches    51418    50360    -1058     
==========================================
- Hits       229810   225978    -3832     
+ Misses      35005    34506     -499     
  Partials     4437     4437              

Flag	Coverage Δ
cli	?
dify-ui	94.75% <ø> (ø)
web	84.85% <ø> (-0.01%)	⬇️

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