fix(setup): root empty-prefix static assets

[linkdata]

Summary

• root direct StaticServe resource URLs so empty-prefix assets resolve from nested pages
• construct owned asset URLs from paths so URL-sensitive filename characters are escaped safely
• preserve page-relative behavior for generic string, URL, and SetupFunc resources
• add nested-page, escaping, wildcard-safety, and compatibility regressions

Verification

• go generate ./...
• go vet ./...
• gofmt and gofumpt
• staticcheck ./...
• golangci-lint run ./...
• gosec ./...
• go build ./...
• JAWS_REQUIRE_NODE=1 go test -race -count=1 -coverprofile=/private/tmp/jaws-163-coverage.out ./...

Closes #163