docs(contributing): switch the frontend CSS guidance to utilities-first (#775)

The "Vanilla CSS … no Tailwind" rule contradicted the (already-wired) Tailwind
v4 setup and the CSS→Tailwind migration plan (#772). Replace it with the
utilities-first standard: Tailwind utilities (bridged to the design tokens via
index.css @theme) for layout/spacing/typography; keep .css files only for the
hard parts (glass, keyframes, pseudo-elements, :has(), theme rules).

Required by the docs-sync rule as P0 of the migration.

Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -159,7 +159,7 @@ class MyEngineBackend(TTSBackend):
 
 - **Components**: Functional components with hooks
 - **State**: Zustand stores in `src/stores/`, organized by slice
-- **CSS**: Vanilla CSS in component-level files — no Tailwind
+- **CSS**: **Utilities-first** — use Tailwind v4 utility classes (mapped to the design tokens in `src/ui/tokens.css` via the `@theme` block in `src/index.css`) for layout, spacing, and typography. Keep component-level `.css` files only for what utilities can't express cleanly: glassmorphism/`backdrop-filter`, `@keyframes`, pseudo-elements, `:has()`, and theme-specific rules. New components should not reflexively spawn a `.css` file. (Migration in progress — see `docs/css-to-tailwind-migration.md`.)
 - **Naming**: `PascalCase` for components, `camelCase` for hooks and utils
 
 ### Rust (Tauri)