Administration: Improve dashboard meta box control visuals

[poligilad-auto]

Trac ticket: https://core.trac.wordpress.org/ticket/50699
Follow-up to: #12180

Summary

• Adds right-side spacing to shared postbox header actions, with RTL support.
• Updates shared postbox header action buttons, including the move and collapse controls, to use the standard wp-admin focus ring treatment.
• Refines dashboard-specific empty drop areas to use a 1px dashed border and align with the existing subtle theme-color drag highlight used for meta box drop zones.

Before

After

Scope

The postbox header action spacing and action-button focus styles live in common.css, so they apply to postbox controls generally. The drag highlight is a general meta box drop-zone pattern; this PR only changes the dashboard empty drop area because that is the dashboard-specific part that needed the visual cleanup.

Context

This extracts small visual improvements from the broader meta box reordering behavior work so the behavior PR can remain focused on the Screen Options toggle and reordering logic.

Testing

• git diff --check
• npm exec -- grunt build:css
• PostCSS safe-parser check for src/wp-admin/css/common.css and src/wp-admin/css/dashboard.css
• Focused declaration assertion for the new spacing, RTL, shared focus, border, and drag-fill CSS
• Playwright check on the local classic WooCommerce product editor confirming the collapse and move buttons compute to the same focus border-radius and box-shadow

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[github-actions]

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