phase-4: scss upgrade

ui/src/styles/app.scss

@@ -0,0 +1,11 @@
+// ── Root stylesheet — imports all shared token/pattern partials ───────────
+// Order matters: tokens first (defines CSS custom properties), then reset
+// (uses tokens on body), then layout patterns.
+
+@use 'tokens';
+@use 'reset';
+@use 'layout';
+@use 'section';
+@use 'table';
+@use 'buttons';
+@use 'badges';