Clean up the now-dead LOGO_DARK/logoDark mapping in globalConfig.js
(LOGO_DARK was removed from DashboardController::GLOBAL_CONFIG_KEYS in
rebase phase 1 f17ce89; logoDark was always undefined and no view consumed
it). Produce a proper favicon-sized 512x512 logo_thumbnail.png from the
1000x1000 logo.png instead of the byte-identical copy. No egress, no
functional change outside removing unused state.
