fix(design): a user row answers as a whole card — the hover layer under its badges and buttons, a click anywhere opens the account (E28)

The layer was the name button's own and ended at the row's first badge or button. The card now paints
the B3 layer under its content (isolate + a new z-under level), the name button's hit layer spans the
card, and the actions sit above it.

frontend/src/index.css

@@ -48,6 +48,9 @@
 
   --font-sans: Inter, system-ui, -apple-system, Segoe UI, Roboto, sans-serif;
 
+  /* `under` is not an app layer: a surface's own layer painted under its content, inside that surface's
+     `isolate` stacking context (a whole-card row's hover layer, DESIGN.md State layers). */
+  --z-index-under: -1;
   --z-index-base: 10;
   --z-index-menu: 20;
   --z-index-chrome: 30;