feat(dify-ui): add shared form primitives

[lyzno1]

Summary

• add shared Base UI Form, Field, and Fieldset primitives in dify-ui
• align field anatomy with Dify Design Kit input, label, help text, and error styles
• compose checkbox group usage with Fieldset semantics and regroup form-control stories

Validation

• pnpm --dir packages/dify-ui test
• pnpm --dir packages/dify-ui type-check
• pnpm --dir web test app/components/base/checkbox-list/tests/index.spec.tsx
• pnpm --dir web type-check
• git diff --check

Fixes #36333

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.96%. Comparing base (2d5186f) to head (a6af695).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #36334   +/-   ##
=======================================
  Coverage   85.96%   85.96%           
=======================================
  Files        4455     4458    +3     
  Lines      211543   211564   +21     
  Branches    39527    39536    +9     
=======================================
+ Hits       181847   181876   +29     
+ Misses      26482    26474    -8     
  Partials     3214     3214           

Flag	Coverage Δ
api	85.49% <100.00%> (+<0.01%)	⬆️
dify-ui	94.80% <100.00%> (+0.21%)	⬆️
web	86.56% <100.00%> (+<0.01%)	⬆️

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[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	44.43%	+44.43%
Strict coverage	0.00%	43.96%	+43.96%
Typed symbols	0	22,544	+22,544
Untyped symbols	0	28,499	+28,499
Modules	0	2562	+2,562