refactor(web): normalize z-index layering

[lyzno1]

Summary

• align application dialogs and portalled pickers with the Dify UI overlay contract
• remove redundant high z-index values while preserving feature-local paint order
• replace legacy workflow graph layers with a single semantic nested-element layer and normalize old graph, paste, copy, and snippet paths
• keep the existing RAG floating-panel layer because removing it would let later positioned panel siblings cover the surface

Regression safeguards

• preserve the Agent Settings panel geometry while using the shared Dialog primitive and its theme-aware default backdrop
• preserve the DSL warning tint paint order without a numeric z-index
• retain React Flow ordering: selected containers resolve to 1000, nested nodes/edges/labels and the default connection line resolve to 1001
• derive inserted snippet edge layers from the nodes final parent relationships, including roots moved into an Iteration or Loop
• keep account and integrations close controls visually above their owning content through owner placement and DOM order

Verification

• vpr dify#lint --quiet --fix --prune-suppressions
• pnpm -C web type-check
• 23 focused test files: 271 passed, 1 skipped
• final focused recheck: 29 passed
• default-backdrop follow-up: 9 passed
• nested snippet regression suite: 7 files, 121 passed
• git diff --check
• local Next.js compilation succeeded; interactive browser smoke was blocked because the local API was not running (ECONNREFUSED)

Screenshots

Not available: the local web route requires a running API. The affected paint-order contracts are covered by focused tests and source-level stacking-context review.

Checklist

• I added or updated behavior-focused tests for the changed workflow and dialog contracts.
• I ran frontend lint and type-check validation.
• Documentation update required.

From Codex

[codecov]

Codecov Report

❌ Patch coverage is 80.30303% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.48%. Comparing base (6e5ba18) to head (9aa5dc1).

Files with missing lines	Patch %	Lines
...omponents/workflow/hooks/use-nodes-interactions.ts	13.33%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38796      +/-   ##
==========================================
+ Coverage   85.47%   85.48%   +0.01%     
==========================================
  Files        5025     5025              
  Lines      273457   273456       -1     
  Branches    51259    51249      -10     
==========================================
+ Hits       233729   233760      +31     
+ Misses      35238    35206      -32     
  Partials     4490     4490              

Flag	Coverage Δ
dify-ui	94.61% <ø> (ø)
web	85.17% <80.30%> (+0.02%)	⬆️

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