refactor(ui): decouple CSS dependencies and improve test quality

[lyzno1]

Summary

Preliminary decoupling work for web/app/components/base/ui/ components to prepare for future @dify/ui package extraction. This PR focuses exclusively on removing CSS coupling and improving test quality — no package extraction is performed.

CSS Decoupling

• Button: Inline all 11 @utility definitions from button/index.css into CVA Tailwind classes, remove the CSS file and its globals.css import so the component is fully self-contained.
• ScrollArea: Replace CSS Module (index.module.css) with a plain CSS file using [data-dify-scrollbar] attribute selectors, eliminating CSS Module loader dependency while preserving style isolation.

Test Quality Improvements

• Remove export === BaseXxx.Root identity tests from dialog, popover, tooltip, and number-field specs (implementation coupling, not bridging behavior).
• Remove @/next/link external project dependency from dropdown-menu spec.
• Remove redundant afterEach(cleanup) from button spec (already handled by vitest.setup.ts).
• Remove unused beforeEach(vi.clearAllMocks) from number-field spec.
• Update stale btn-* class name assertions to actual design-token classes in alert-dialog and button specs.
• Mark intentionally public re-exports (DialogTrigger, DialogClose, PopoverTitle, PopoverDescription) with @public JSDoc for knip.

All 13 test files (165 tests) pass. ESLint, TypeScript type-check, and knip all clean.

From Cursor.

Screenshots

No visual changes — CSS refactoring preserves identical rendered output.

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.85%. Comparing base (3bccdd6) to head (d620e31).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35242      +/-   ##
==========================================
- Coverage   84.85%   84.85%   -0.01%     
==========================================
  Files        4360     4360              
  Lines      196685   196683       -2     
  Branches    37281    37280       -1     
==========================================
- Hits       166900   166895       -5     
- Misses      26703    26706       +3     
  Partials     3082     3082              

Flag	Coverage Δ
web	85.76% <100.00%> (-0.01%)	⬇️

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