Fix favicon and mobile email header

[josephiesue]

What changed

• Replaced the favicon's external image dependency with self-contained SVG geometry.
• Added scalable favicon metadata and a cache-busting URL.
• Locked the Lead Letter email logo group to a white background with dark text in the weekly and welcome templates.
• Added light-only color-scheme metadata to the welcome email.

Why

The favicon SVG referenced a separate image that browsers do not reliably load inside favicon documents. Mobile email clients could also preserve the light header gradient while independently changing the adjacent brand text to white, making the label disappear.

Impact

The site favicon should render reliably after deployment, and newly generated SendFox emails should keep the Lead Letter header label readable in mobile dark mode.

Validation

• Astro production build completed successfully (20 pages).
• Generated weekly and welcome email HTML contains the background and text-color protections.
• Mobile-width browser preview rendered the brand label as dark ink on white with no relevant console errors.
• JavaScript syntax checks and git diff --check passed.