fix: auto-load theme1 with mask+metrics on first install (v9.2.1)

After theme archives finish extracting, _update_theme_directories now
auto-loads the first local theme onto LCD + preview when current_image
is None (first install / fresh config). overlay_config=True ensures the
theme's default config1.dc (mask + metrics) is applied, not just the
bare background.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>