Redesign the setup flow and consolidate the CSS into one primitives layer

Onboarding asks explicitly for one or two parents and shows one card per
parent for balances; the balance form is per parent with inline
validation; the partner can be filled from the FK deduction or added
from it. Settings and Dashboard reuse the same form.

CSS: bare buttons carry no colour any more; buttons, cards, choice
cards, fields, alerts, badges, eyebrow and modal chrome are defined once
in primitives.css. Media queries live only in responsive.css. A style
guard test blocks hex, !important and stray media queries. Bilingual
fixes: calendar selects size by content, header no longer wraps. Vitest
is capped at four workers.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

frontend/src/components/layout/layout.css

@@ -23,6 +23,10 @@
   width: 100%;
 }
 
+.dashboard > .alert {
+  margin-bottom: var(--sp-5);
+}
+
 /* Sections separate by whitespace and a hairline rule, not a stack of
  * bordered/shadowed cards — every section on a page looking equally "boxed"
  * flattens the hierarchy the headline figures above them are meant to have.
@@ -64,15 +68,6 @@
   margin-top: 0;
 }
 
-.dashboard__error {
-  padding: var(--sp-4);
-  background-color: var(--c-danger-weak);
-  color: var(--c-danger);
-  border-left: 4px solid var(--c-danger);
-  border-radius: var(--radius);
-  margin-bottom: var(--sp-5);
-}
-
 .dashboard__break-list {
   list-style: none;
   margin: 0;
@@ -111,17 +106,3 @@
   border-top: 1px solid var(--c-border);
 }
 
-/* Responsive */
-@media (max-width: 640px) {
-  [data-testid='layout-main'] {
-    padding: var(--sp-4);
-  }
-
-  .dashboard__section,
-  .settings-page__section,
-  .calendar__section,
-  .tips-page__section {
-    padding: var(--sp-4);
-    margin-bottom: var(--sp-4);
-  }
-}