fix: reorganize Gitea customizations to proper directory structure

- Move themes to dev/custom/public/assets/css/ (correct Gitea custom path)
- Add custom favicon.svg and logo.svg to dev/custom/public/assets/img/
- Update volume mount to use /data/gitea/custom instead of incorrect path
- Follows Gitea customization documentation structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

dev/compose.yaml

@@ -11,7 +11,7 @@ services:
       - gitea_config:/etc/gitea
       - /etc/timezone:/etc/timezone:ro
       - /etc/localtime:/etc/localtime:ro
-      - ./dev/themes:/data/gitea/public/assets/css:ro
+      - ./dev/custom:/data/gitea/custom:ro
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       USER_UID: 1000