resolveDefaultSurface was using ListEnabledByType("surface") which
missed extension packages with type "full". Switch to List() and
filter by source != core/builtin + type surface/full.
Also make welcome surface smarter: detect when extensions exist
but no default is set vs truly empty install, show appropriate
messaging and CTA (Set Default Surface vs Go to Packages).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>