[@mantine/core] MantineProvider: Fix topmost pixel click issue on active elements

[saadpocalypse]

Problem

Clicking on the very topmost pixel of a button, action icon, or other interactive element using the .mantine-active class shows the pointer cursor, but fails to register a click event.

This occurs because:

• When active, .mantine-active applies transform: translateY(rem(1px)) to visual elements to create a tactile "pressed" effect.
• Translating the element down shifts its bounding box 1px down.
• If a user initiates a click on the topmost pixel of the element, the translation moves the element's top boundary out from under the mouse cursor.
• Upon mouseup, the cursor is no longer within the element's boundaries, causing the browser to abort the click event.

Solution

Refactored the .mantine-active styles in packages/@mantine/core/src/core/MantineProvider/global.css:

• Temporarily allowed overflow (overflow: visible) during the :active state.
• Appended a 1px tall transparent ::after pseudo-element positioned at top: rem(-1px) spanning the full width of the element.
• This pseudo-element bridges the gap and keeps the hover/click target active relative to the cursor's original position when the element shifts down.
• Explicitly hidden/disabled the pseudo-element in fieldset:disabled to prevent unwanted interactions.

Tests

• Style Checks: Verified CSS rules conform to style guidelines using npm run stylelint and formatted the file with npm run format:write:files.
• Build & Typecheck: Successfully verified that the codebase typechecks (npm run typecheck) and builds without errors (npm run build).
• Manual Verification: Tested on Storybook across Button and ActionIcon stories to verify that clicking the topmost pixel triggers events correctly and triggers the tactile visual press animation seamlessly.

Resolves 9064