refactor(frontend): retire legacy CSS and isolate visual artwork

AGENTS.md

@@ -167,8 +167,10 @@ evidence; do not bundle them into tooling or structural refactors.
   `vocero-frontend/src/lib/messages.ts`. User text stays literal, and exports retain
   raw message text. Keep sanitization and unsafe-link regression coverage when
   changing it.
-- Reuse the existing design tokens, Tailwind integration, and
-  `vocero-frontend/src/styles/legacy-theme.css`. Verify both light and dark themes,
+- Reuse the Tailwind tokens in `vocero-frontend/src/styles/theme.css` and shared
+  controls in `vocero-frontend/src/components/ui.tsx`. Keep specialized artwork
+  and sanitized Markdown styles scoped under `vocero-frontend/src/styles/`.
+  Verify both light and dark themes,
   narrow screens, keyboard focus, dialog dismissal, and voice-state feedback for
   UI changes.
   Preserve intentional `?debug=true` controls and browser audio-permission flows.