refactor(dify-ui): render ContextMenu story trigger as a semantic area

[Copilot]

Summary

Per Base UI docs, ContextMenu.Trigger is semantically an area (right-click / long-press) that renders a <div> by default — unlike Menu.Trigger, which is a click-activated button. The ContextMenu story wrongly overrode the trigger with render={<button type="button" />}, giving it incorrect button semantics (implicit role="button", keyboard activation, extra style reset burden) that mismatch a right-click region.

Changes

• packages/dify-ui/src/context-menu/index.stories.tsx: TriggerArea now applies styling via className directly on ContextMenuTrigger, letting it render its default <div> area instead of a <button>. Visual appearance (size, border, background, focus ring) is preserved; only the button element and type="button" are removed.

<ContextMenuTrigger
  aria-label="context menu trigger area"
  className="flex h-44 w-80 items-center justify-center rounded-xl border ..."
>
  {label}
</ContextMenuTrigger>

Story-only, non-behavioral change; existing tests already relied on the default <div> rendering.

Screenshots

Before	After
Trigger rendered as <button>	Trigger rendered as default <div> area

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

From Copilot

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.30%. Comparing base (f755830) to head (f1983d6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #38420   +/-   ##
=======================================
  Coverage   85.30%   85.30%           
=======================================
  Files        4995     4995           
  Lines      264326   264326           
  Branches    50141    50141           
=======================================
  Hits       225479   225479           
  Misses      34429    34429           
  Partials     4418     4418           

Flag	Coverage Δ
dify-ui	94.75% <ø> (ø)
web	84.86% <ø> (ø)

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