feat: accessibility testing suite + fix 5 WCAG AA contrast failures

[Copilot]

No accessibility-targeted tests existed. This adds WCAG 2.1 color-contrast validation, automated axe-core component audits, and keyboard/ARIA interaction tests — and fixes the real contrast failures those tests uncovered in the light theme.

CSS fixes (5 WCAG AA violations)

Token	Before	After	Ratio
--alert-error-text on #fef2f2	#dc2626	#b91c1c	4.41 → 5.91
--alert-warning-text on #fffbeb	#d97706	#92400e	3.07 → 6.84
--alert-success-text on #f0fdf4	#16a34a	#166534	3.15 → 6.81
White on toast-success-bg #10b981 (light)	white	#111827	2.54 → 6.99
White on toast-warning-bg #f59e0b (light)	white	#111827	2.15 → 8.26

Dark theme already passed; no dark-mode changes needed.

New: src/test/colorContrast.ts

WCAG 2.1 utility library — relativeLuminance, contrastRatio, meetsWcagAA/AAA, and the WCAG threshold constants. No dependencies; pure algorithm.

New: src/test/colorContrast.test.ts — 57 tests

• Algorithm unit tests (sRGBToLinear, hexToRgb, relativeLuminance, contrastRatio, meetsWcagAA)
• 9 spot-check ratio assertions keyed to known values (regression guard)
• Every design-token foreground/background pair for both light and dark themes, each asserted at the appropriate AA threshold (4.5:1 normal text, 3:1 large text / UI components)

New: src/test/accessibility.test.tsx — 68 tests

Three layers:

1. Automated axe-core audits — all 17 shared components rendered and checked with toHaveNoViolations()
2. Semantic / ARIA — role="status", aria-live="polite", heading levels, label association, name grouping on radio groups, required indicators
3. Keyboard interaction — Escape dismissal (Modal, ConfirmDialog, ErrorDialog), Enter/Space activation (Button, Toggle, PillToggle, ViewModeSelector), ArrowDown navigation (RadioGroup), overlay-click dismissal

it('fires onChange when ArrowDown moves to the next radio option', async () => {
  render(<RadioGroup name="letters" value="a" options={options} onChange={vi.fn()} />);
  screen.getByLabelText('Alpha').focus();
  await userEvent.keyboard('{ArrowDown}');
  expect(screen.getByLabelText('Beta')).toHaveFocus();
});

Infrastructure

• src/test/setup.ts — registers jest-axe toHaveNoViolations matcher via expect.extend
• src/test/jest-axe.d.ts — vitest Matchers augmentation for toHaveNoViolations (avoids pulling in @types/jest-axe which carries a conflicting jest type reference)

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