fix(web): respect marketplace feature flag in model selector

[lyzno1]

Summary

• read existing system features in the model selector popup and gate marketplace UI on enable_marketplace
• disable marketplace plugin queries from the model-provider helper when the feature flag is off
• keep installed/configured models visible while hiding marketplace install prompts and AI-credit marketplace fallbacks

Fixes #36881

Tests

• pnpm --dir web test app/components/header/account-setting/model-provider-page/tests/hooks.spec.ts app/components/header/account-setting/model-provider-page/model-selector/tests/popup.spec.tsx
• pnpm --dir web type-check

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.82%. Comparing base (240912c) to head (4141704).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...eader/account-setting/model-provider-page/hooks.ts	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #36883      +/-   ##
==========================================
- Coverage   85.82%   85.82%   -0.01%     
==========================================
  Files        4553     4553              
  Lines      222925   222941      +16     
  Branches    41006    41016      +10     
==========================================
+ Hits       191332   191344      +12     
- Misses      27897    27901       +4     
  Partials     3696     3696              

Flag	Coverage Δ
dify-ui	95.57% <ø> (ø)
web	86.50% <90.90%> (-0.01%)	⬇️

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