Add styled HTML layout for mailer

[CtrlAltDefeat17]

What was done

The mailer layout (app/views/layouts/mailer.html.erb) was previously an unstyled Rails default , which caused mails to look plain.

This PR replaces it with a styled, table-based HTML layout:

• 600px wide, responsive, Outlook-compatible (inline CSS, VML fallbacks)
• Header with SciNote branding, content area, and footer
• Footer optionally shows the instance URL from WEB_SERVER_URL

All texts are i18n-based; three new keys were added under mailer.layout in config/locales/en.yml.

Note:

• The accent color was chosen to match the SciNote UI, but feel free to adjust it to the exact brand value.