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/common/loading-spinner.css

@@ -10,7 +10,7 @@
 }
 
 .loading-spinner__icon {
-  font-size: 48px;
+  font-size: var(--icon-lg);
   color: var(--c-accent);
   animation: spin 2s linear infinite;
   display: block;
@@ -30,10 +30,3 @@
     transform: rotate(360deg);
   }
 }
-
-/* Reduce Motion */
-@media (prefers-reduced-motion: reduce) {
-  .loading-spinner__icon {
-    animation: none;
-  }
-}