fix(a11y): add ARIA attributes to interactive elements

- ImageBlock: wrap clickable image in <button> with aria-label, add
  role=dialog and aria-modal to lightbox overlay
- KeyboardShortcuts: add role=dialog and aria-modal to modal overlay
- CodeBlock: add aria-label to copy button
- LanguageSelector: add aria-label with current language

src/components/KeyboardShortcuts.tsx

@@ -50,7 +50,7 @@ export function KeyboardShortcuts({ open, onClose }: Props) {
   if (!open) return null;
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center" onClick={onClose}>
+    <div role="dialog" aria-modal="true" aria-label={t('shortcuts.title')} className="fixed inset-0 z-50 flex items-center justify-center" onClick={onClose}>
       {/* Backdrop */}
       <div className="absolute inset-0 bg-black/60 backdrop-blur-sm" />