Spork Android android-20260703234106

fix(a11y): resolve all WCAG A/AA violations found by axe-core

Ran axe-core across every screen and fixed the real findings — now ZERO
violations on all 18 (games, lists, auth):
- meta-viewport: removed maximum-scale/user-scalable=no (blocked pinch-zoom,
  WCAG 1.4.4); kept viewport-fit=cover for the safe-area insets.
- color-contrast: --sp-text-muted #6b7280→#646b78 (4.47→4.96), and --sp-accent
  #5b8def→#3f6fd6 so white button text clears 4.5:1 (was 3.23) app-wide; text-on-
  light uses (tab label, auth link, .sp-kicker) point at --sp-accent-strong.
  Home card gradients use their own literal hex, unaffected.
- button-name: the 64 chess squares now have aria-labels ('e4, white pawn' /
  'a1, empty') via a pure squareLabel() helper (+tests).
- role-img-alt: decorative IonIcons (deck-card placeholder, category chevrons)
  marked aria-hidden.
Verified with a full axe sweep: 0 violations across all screens.