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.test.tsx

@@ -55,6 +55,8 @@ describe('Layout', () => {
       personalDataUuid: null,
       parentName: null,
       refreshContext: vi.fn(),
+      planVersion: 0,
+      bumpPlanVersion: vi.fn(),
     })
   })
 
@@ -86,6 +88,8 @@ describe('Layout', () => {
       personalDataUuid: 'pd1',
       parentName: 'Elin',
       refreshContext: vi.fn(),
+      planVersion: 0,
+      bumpPlanVersion: vi.fn(),
     })
     renderLayout()
     expect(screen.getByTestId('parent-name')).toHaveTextContent('Elin')
@@ -108,6 +112,8 @@ describe('Layout', () => {
       personalDataUuid: 'pd1',
       parentName: null,
       refreshContext: vi.fn(),
+      planVersion: 0,
+      bumpPlanVersion: vi.fn(),
     })
     renderLayout()
     expect(screen.getByDisplayValue('Emma')).toBeInTheDocument()