refactor(web): converge overlay layering on dify-ui z-50

[lyzno1]

Summary

• converge dify-ui overlay primitives on the shared z-50 layer and keep toast at z-60
• migrate remaining drawer-plus callers to @langgenius/dify-ui/drawer and remove drawer-plus
• replace the migration guide with the current overlay best-practice guide
• remove stale high z-index overlay assertions and keep nested drawer dismissal/backdrop behavior wired through Base UI

[codecov]

Codecov Report

❌ Patch coverage is 71.24183% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.82%. Comparing base (8581a68) to head (916b055).

Files with missing lines	Patch %	Lines
...nents/tools/edit-custom-collection-modal/index.tsx	60.00%	18 Missing ⚠️
...ents/tools/setting/build-in/config-credentials.tsx	42.85%	8 Missing ⚠️
...nts/app/annotation/edit-annotation-modal/index.tsx	76.19%	5 Missing ⚠️
...ents/app/annotation/add-annotation-modal/index.tsx	50.00%	4 Missing ⚠️
...nts/app/annotation/view-annotation-modal/index.tsx	73.33%	4 Missing ⚠️
...ts/tools/edit-custom-collection-modal/test-api.tsx	60.00%	4 Missing ⚠️
web/service/tools.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35976      +/-   ##
==========================================
- Coverage   85.82%   85.82%   -0.01%     
==========================================
  Files        4454     4453       -1     
  Lines      209796   209795       -1     
  Branches    39295    39294       -1     
==========================================
- Hits       180067   180047      -20     
- Misses      26514    26533      +19     
  Partials     3215     3215              

Flag	Coverage Δ
dify-ui	94.40% <100.00%> (ø)
web	86.62% <71.05%> (-0.03%)	⬇️

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[CodingOnStar]

LGTM