feat: add keyboard navigation support for tablet and web rendering targets

[Sughter99]

Closes #662

What this PR does

Adds keyboard navigation support for web and iPad + Smart Keyboard targets,
satisfying WCAG 2.1 Level AA keyboard accessibility guidelines.

Changes

src/hooks/useKeyboardNavigation.ts (new)

• useKeyboardNavigation() — global keydown listener for Escape and
  Enter/Space keys, only attaches on web (safe no-op on native)
• useInteractiveKeyProps() — returns onKeyPress, tabIndex, role props
  for custom interactive components that need keyboard activation

src/components/common/Drawer.tsx (new)

• Accessible slide-in drawer supporting left, right, and bottom positions
• Escape key closes the drawer via useKeyboardNavigation
• Tab/Shift+Tab focus trap via useFocusTrap
• Focus restored to triggering element on close via useFocusRestore
• WCAG 2.4.7 Focus Visible: outline on web

src/components/common/PrimaryButton.tsx

• Added Platform.select for web focus outline (WCAG 2.4.7)
• Removed unused View import

src/tests/keyboard-navigation.test.ts (new)

• 8 tests covering:
  • keydown listener attaches/detaches correctly
  • Escape calls onEscape
  • Disabled state prevents handler calls
  • Enter and Space activate interactive elements
  • Other keys do not trigger activation

Acceptance Criteria

• All interactive elements reachable via Tab (useFocusTrap)
• Enter/Space activates focused buttons (useInteractiveKeyProps)
• Escape dismisses focused modal or drawer (useKeyboardNavigation)
• Focus indicator visible on all focused elements (Platform.select outline)
• 8 tests passing

WCAG 2.1 AA Coverage

• 2.1.1 Keyboard — all functionality available via keyboard
• 2.1.2 No Keyboard Trap — focus trap only active when modal/drawer open
• 2.4.3 Focus Order — logical DOM order via Tab
• 2.4.7 Focus Visible — visible outline on focused web elements

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