Optimize the header logo and favicon payload

[n30nex]

Summary

The same large PNG is used as both the header logo and favicon on every page. It is much larger than either rendered use requires.

Audit target: production revision 578e7d1cdcbeeb03a9946b39e7fdfc2f932dae33 on 2026-08-29.

Evidence

• docs/assets/MeshCore-Canada-Favicon.png is a 554 × 612 RGBA image and is 274,340 bytes in the repository.
• mkdocs.yml uses that file for both theme.favicon and theme.logo.
• Live Lighthouse network inventories transferred about 274.7 KB for this image on both the English and French homepages.
• The full homepage transfer was about 525–526 KB in the same runs, so the logo/favicon accounts for over half of the transferred bytes.
• In the header it is rendered at only a few dozen CSS pixels.

Expected result

Use separate, appropriately sized assets for the header logo and browser favicon. Prefer an existing vector source for the header if one is available; otherwise export optimized raster sizes with enough resolution for high-density displays.

Acceptance checks

• The header remains sharp in light/dark themes and on high-density screens.
• The favicon remains recognizable in supported browsers.
• The combined first-page transfer for logo/favicon assets is substantially below the current ~275 KB.
• English and French layouts, accessible names, and existing visual tests remain unchanged.