[@mantine/core] Drawer/Modal: Add Footer components

[saadpocalypse]

Problem

Users currently struggle to implement fixed footer content (such as action buttons) at the bottom of a Drawer or Modal when the main body content (Drawer.Body / Modal.Body) is scrollable.
Common workarounds (like reusing <Drawer.Header bottom={0}>Footer</Drawer.Header> or using custom absolutely positioned containers) are hacky, break HTML semantics (using a <header> tag for a footer), and require manual layout overrides that conflict with style rules. There was no native support for a sticky footer layout.

Solution

• Introduced ModalBaseFooter in packages/@mantine/core/src/components/ModalBase rendering a semantic <footer> element.
• Added a .footer CSS class to ModalBase.module.css with sticky positioning (position: sticky; bottom: 0;), background color support, and standard padding spacing.
• Created and registered the Drawer.Footer component using ModalBaseFooter with support for Drawer styles API.
• Created and registered the Modal.Footer component using ModalBaseFooter with bottom corner border-radius variables (border-end-start-radius / border-end-end-radius) configured in Modal.module.css.
• Documented the footer selector inside the Drawer and Modal styles API definitions.
• Added WithFooter story test cases in both Drawer.story.tsx and Modal.story.tsx for easy manual verification.

Tests

• Automated tests: Ran npm run jest packages/@mantine/core/src/components/Drawer/Drawer.test.tsx packages/@mantine/core/src/components/Modal/Modal.test.tsx — 556 tests passed successfully.
• Build, typecheck & formatting: Verified clean output for npm run typecheck, npm run build, npm run oxlint, and npm run stylelint.
• Manual verification: Checked Storybook → Drawer → With Footer and Storybook → Modal → With Footer.

Resolves #4540 and #8680