fix(web): normalize dify-ui overlay control

[lyzno1]

Summary

• Normalize touched dify-ui overlay call sites to controlled open / onOpenChange usage instead of gating overlay content with JSX conditionals.
• Keep create subscription modal state controlled with isCreateModalOpen; move modal business state into DialogContent so Base UI owns content mount/unmount during close animations.
• Remove per-call-site z-index overrides from @langgenius/dify-ui/* overlay call sites, including popupClassName="z-*", positionerProps.style.zIndex, and the variable reference wrapper zIndex escape hatch. Legacy PortalToFollowElem call sites are intentionally left untouched.
• Update the create subscription modal mock to respect controlled open=false.

Docs checked

• Base UI Dialog docs: controlled usage is open + onOpenChange; manual unmount / keepMounted / actionsRef is for JavaScript animation control.
• web/docs/overlay-migration.md: new dify-ui overlays should not add per-call-site z-index overrides.
• packages/dify-ui/AGENTS.md: overlay primitive semantics, z-index contract, and accessibility expectations.
• Vercel React / Web Interface Guidelines: explicit conditional rendering, visible focus/accessibility, no unnecessary state effects.

Verification

• pnpm type-check
• pnpm lint:tss
• pnpm test app/components/apps/__tests__/app-card.spec.tsx app/education-apply/__tests__/search-input.spec.tsx app/components/plugins/plugin-detail-panel/subscription-list/create/__tests__/common-modal.spec.tsx app/components/plugins/plugin-detail-panel/subscription-list/create/__tests__/index.spec.tsx
• Targeted ESLint on changed files: 0 errors; remaining output is existing warnings.
• Precise scan for dify-ui overlay z-index overrides: 0 matches.

[codecov]

Codecov Report

❌ Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.81%. Comparing base (03e227f) to head (3d2d645).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
web/app/components/apps/app-card.tsx	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #35832   +/-   ##
=======================================
  Coverage   85.81%   85.81%           
=======================================
  Files        4459     4459           
  Lines      208987   208994    +7     
  Branches    39063    39062    -1     
=======================================
+ Hits       179343   179351    +8     
+ Misses      26477    26476    -1     
  Partials     3167     3167           

Flag	Coverage Δ
dify-ui	93.27% <ø> (ø)
web	86.76% <98.21%> (+<0.01%)	⬆️

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