fix: use leaflet.pub fallback URL for docs without basePath
Documents without a custom subdomain (basePath) were falling back to
pdsls.dev, but leaflet.pub supports a /p/did/rkey URL pattern that
provides a native leaflet viewing experience.
Now uses leaflet.pub/p/{did}/{rkey} instead of pdsls.dev for leaflet
documents missing basePath.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>