style: color What's New tags by category

[nate-appflowy]

Summary

• Set What's New version and category tag text to 12px.
• Match tag colors to the Figma reference: purple for version, pink for new features, blue for improvements, and green for bug fixes.
• Set the page subtitle to 16px and #5a5a5a.
• Let the version metadata and tag area use the full available width below 640px; preserve the desktop width rule.
• Change the dividers between news items to light gray (#E6E6E6).

Verification

• Reviewed the changes and ran git diff --check before committing.
• Tests were not run.