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/LanguageSelector.tsx

@@ -16,6 +16,7 @@ export function LanguageSelector() {
       onClick={cycle}
       className="flex items-center gap-1.5 rounded-2xl border border-white/8 bg-zinc-800/30 px-2.5 py-1.5 text-xs text-zinc-400 hover:text-zinc-200 hover:bg-white/5 transition-colors"
       title="Change language"
+      aria-label={`Language: ${localeLabels[current] || current}. Click to change.`}
     >
       <Globe size={14} />
       <span className="font-medium">{localeLabels[current] || current.toUpperCase()}</span>