Link UI: Introduce margin to fix obscuring the + button while editing navigation sub-menus

[yogeshbhutkar]

Fixes: #68182

What, Why & How?

Currently, when editing navigation links, the link UI popover can obscure the "+" button, making it difficult for users to add new items. This PR adds a top margin to the popover to ensure better visibility and accessibility of the add button.

• Add top margin to Navigation Link block popover
• Use existing block margin variable for consistency
• Improve UX for navigation menu editing

Testing Instructions

1. Create a navigation block
2. Try adding sub-menu items to menu items
3. Notice, the + button now does not overlap with the Popover

Screenshots

Before	After

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: getdave <[email protected]>

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[yogeshbhutkar]

Closing the PR as the solution proposed here is not relevant.