Global Elements: Add umb-entity-frame component for entity references

[leekelleher]

Description

As part of the Global Elements phase 2 work (Reusable Content of Blocks, see #22448), we need UI to distinguish a referenced global element from a local block. Introducing an <umb-entity-frame> component brings us a step closer towards this.

Summary (AI/Claude generated 🤖)

• Adds a new umb-entity-frame component under src/packages/core/components/ — a passive overlay that frames its parent with a rounded border and label tab.
• Visibility is controlled by the consumer via the --umb-entity-frame-opacity CSS custom property (defaults to 1); typical pattern is for the parent container to set it to 0 and toggle to 1 on :hover and/or :focus-within.
• Exposes three CSS custom properties: --umb-entity-frame-opacity, --umb-entity-frame-color, and --umb-entity-frame-border-width.
• Includes Storybook stories (8 variants) and unit tests following established core component conventions.

Test Plan

• Storybook: navigate to Generic Components/Entity Frame and verify all 8 stories render correctly
• Verify hover-based stories (OnHover, OnHoverOrFocus, WrappingButton, Nested) toggle visibility as expected
• Verify WithCustomColor story renders with the purple theming (#7532c8)
• Verify WithSlot story shows the slotted icon + text instead of the label fallback
• Run unit tests: npm test -- --files "**/entity-frame.test.ts"
• No new TypeScript errors: npx tsc --noEmit

[claude]

Claude finished @leekelleher's task in 3m 56s —— View job

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Review posted. 3 suggestions filed; no critical or important issues found. Approved with Suggestions for improvement.

[leekelleher]

Closing this PR, superseded by #22844 for v17.6