Phase 3b of the Search/Artists unification. The Search page's
internal id was 'downloads', which clashed with the actual Downloads
page (id 'active-downloads') and confused anyone reading the code.
Renamed to 'search' across HTML, navigation, DOM selectors, and the
deep-link route list.
Backwards compat: navigateToPage('downloads') aliases to 'search'
at the top of the function; /downloads URL still serves index.html
and the client router resolves the page correctly; profile ACL
checks accept both 'search' and 'downloads' so existing profiles
with 'downloads' in allowed_pages keep working without migration.
Sidebar label unchanged. Zero visual change — pure internal tidy.
