Define the portable Wippy frontend contract

[AndrewKirkovski]

Summary

• make PrimeVue-theme-first portability the canonical frontend contract
• distinguish runtime-backed theme semantics from compile-time Tailwind baselines
• document source-backed theme switching through host.setThemeMode, @theme, AppConfig, and host.getThemeMode
• define build ownership, import-map, casing, custom-composite, and unsupported project-bound rules
• add generated token/Tailwind catalogues and fail-closed documentation/visual checks

Verification

• frontend documentation publication gate passed
• generated catalogues match the selected @wippy-fe/theme source
• commit-bound visible-browser evidence checker passed
• final cold review found no remaining blockers