PublicURL() copied only the scheme and host from conf.Server.ShareURL,
silently dropping any path component. This broke OpenGraph image URLs
(and other share links) when ShareURL was configured with a path prefix
like https://example.com/navi — generated URLs pointed to /share/img/...
at the root instead of /navi/share/img/...
Now the ShareURL path is prepended to the resource path, with trailing
slashes trimmed. When ShareURL has no path, behavior is unchanged.