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/dashboard/PlanFold.tsx

@@ -113,7 +113,7 @@ export function PlanFold({ workspaceId, members }: PlanFoldProps): ReactNode {
         )}
 
         {proposalError && (
-          <div className="plan-fold__error" role="alert" data-testid="plan-fold-proposal-error">
+          <div className="alert alert--error" role="alert" data-testid="plan-fold-proposal-error">
             {proposalError}
           </div>
         )}