[brand] remove dead LOGO_DARK mapping; resize logo_thumbnail to 512x512

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.

app/javascript/shared/store/globalConfig.js

@@ -17,7 +17,6 @@ const {
   INSTALLATION_NAME: installationName,
   LOGO_THUMBNAIL: logoThumbnail,
   LOGO: logo,
-  LOGO_DARK: logoDark,
   PRIVACY_URL: privacyURL,
   IS_ENTERPRISE: isEnterprise,
   TERMS_URL: termsURL,
@@ -48,7 +47,6 @@ const state = {
   hCaptchaSiteKey,
   installationName,
   logo,
-  logoDark,
   logoThumbnail,
   privacyURL,
   termsURL,