[codex] Add favicon assets from header logo

[nickmoreton]

Summary

• add a header-logo-based SVG favicon source asset
• generate SVG, 32x32 PNG, and Apple touch icon outputs through the existing image pipeline
• wire favicon links into the base template
• move already-merged production-mode changelog entries plus this favicon entry into the 2026-04-28 release section

Production deployment prep

• changelog is dated for 2026-04-28
• no migrations or environment changes required
• compiled favicon assets are generated by the existing frontend build and remain untracked

Checks

• npm run biome:check
• npm run build
• uv run python manage.py findstatic img/favicon.svg img/favicon-32x32.png img/apple-touch-icon.png

Note: npm run build still reports existing Sass deprecation warnings.