Add a Branding section to Site Administration where admins can upload
custom images for three separate slots:
- Nav icon (logo-small.png) — top-left corner, 30x30px
- Login logo (logo.png) — login page and homepage
- Favicon (favicon.png) — browser tab icon
Changes:
- New admin route: /-/admin/branding with upload forms
- Templates use AssetUrlPrefix instead of hardcoded external URLs
- Nav bar uses logo-small.png with fallback to logo.png
- Uploads save to custom/public/assets/img/ (persists across restarts)
- SVG overrides auto-removed when PNG is uploaded
- Added logo-small.png as default built-in asset
Closes#181
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>