fix(a11y): fix primary button contrast ratio + tap-target test false positives

- Fix --sumi-text-inverse: #13110f → #f5f0e8 (was dark-on-dark)
  Primary buttons now have ~4.8:1 contrast ratio (WCAG AA pass)
  Affects: Sign In, Register, all primary action buttons

- Tap-target test: skip sr-only elements (intentionally invisible)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/index.css

@@ -40,7 +40,7 @@
   --sumi-text-secondary: #9e9688;
   --sumi-text-tertiary:  #6b6560;
   --sumi-text-disabled:  #3d3930;
-  --sumi-text-inverse:   #13110f;
+  --sumi-text-inverse:   #f5f0e8;
   --sumi-text-link:      #b83a1e;
 
   /* ═══ PIGMENTS — Shu vermillion (朱) + Kin gold (金) ═══ */

tests/e2e/audit/pixel-perfect/12-tap-targets.spec.ts

@@ -17,6 +17,9 @@ test.describe('TAP TARGETS — Chaque élément cliquable fait au moins 44×44px
           if (rect.width === 0 || rect.height === 0) return;
           const style = getComputedStyle(el);
           if (style.display === 'none' || style.visibility === 'hidden') return;
+          // Skip screen-reader-only elements (intentionally invisible)
+          if (style.position === 'absolute' && style.overflow === 'hidden' && (rect.width <= 1 || rect.height <= 1)) return;
+          if (el.classList.contains('sr-only')) return;
           // Skip éléments hors viewport
           if (rect.bottom < 0 || rect.top > window.innerHeight) return;