Resolve FileRoot per storage backend to keep object keys host-independent

LoadConfig unconditionally rewrote a relative FileRoot to an absolute
path under the config dir. R2FileStore.key() then embedded that local
path in every object key (e.g. Users/jhalter/.../config/Files/...), so
bucket contents were tied to the host's directory layout and orphaned
by moving the config dir or pointing another server at the bucket.

FileRoot is a path within the selected file store's namespace, so only
the backend selection in main knows how to resolve it: the os backend
resolves relative values against the config dir as before, while the
memory and r2 backends keep the configured value verbatim, yielding
portable keys like Files/foo.txt. An absolute FileRoot combined with an
object-store backend now logs a warning.

WithConfig copies the config struct, so the option is appended after
the backend selection mutates FileRoot rather than before.

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