fix(notifications): update GlobalAlert box-shadow to inset

[jzempel]

Description

The original GlobalAlert was designed to support stacking by defining a bottom border. The implementation used box-shadow to avoid affecting size. However, without inset the "border" was cut off. This PR fixes GlobalAlert to honor the original design intent.

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer)
• 🌐 demo is up-to-date (npm start)
• ⬅️ renders as expected with reversed (RTL) direction
• ⚫ renders as expected in dark mode
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance
• 📝 tested in Chrome, Firefox, Safari, and Edge

[coveralls]

coverage: 95.511%. remained the same
when pulling 1cee41c on jzempel/global-alert-shadow
into 99e953d on main.