feat: improve email templates with dark mode and styling

[maartenbode]

Summary

• Add dark mode support for email templates with prefers-color-scheme media query, including dark logo swap
• Publish notification email template and remove default "Hello!" greeting (notifications with explicit ->greeting() still render theirs)
• Update light mode background color to #FAFAF8
• Replace inner-body box shadow with subtle border and 8px border-radius
• Switch body text and button font size to 14px
• Flip sign up / log in buttons on invitation accept page (sign up as primary action)