Lets a user pick a sub-folder under their library as a working index root,
stored server-side (new sidecar user_prefs table). The Library tree, reindex,
and both duplicate views (stacks + cross-folder scan) now re-root to it via a
single userLibraryBase() helper. Also fixes the cross-folder scan/archive
endpoints, which previously walked/touched the whole originals root instead
of being scoped per-user (archive now rejects out-of-scope paths, 403).
Removes PhotoPrism settings (Search/Maps/Server-UI/Features/Import) that only
steered PhotoPrism's own bundled SPA and were never read by mulimage's UI.
Also fixes the Library tree occasionally getting stuck on "Loading folders…"
by dropping gcTime:0 and gating the spinner on isLoading instead of isPending.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
