fix: localize UI to English and add ARIA accessibility attributes

- Replace all French UI strings with English (Connected, Thinking, Result, Send, etc.)
- Add role="log" + aria-live="polite" to chat message area
- Add role="form" + aria-label to message input area
- Add aria-label to sidebar toggle, attach file, send, and textarea
- Add role="banner" to header, role="navigation" to sidebar
- Add role="application" to app root

src/components/ThinkingBlock.tsx

@@ -11,7 +11,7 @@ export function ThinkingBlock({ text }: { text: string }) {
         className="inline-flex items-center gap-1.5 rounded-2xl border border-white/8 bg-zinc-800/35 px-3 py-1.5 text-xs text-violet-300 hover:bg-white/5 transition-colors"
       >
         <Brain size={13} />
-        <span className="font-medium">Réflexion</span>
+        <span className="font-medium">Thinking</span>
         {open ? <ChevronDown size={12} className="ml-1 text-zinc-500" /> : <ChevronRight size={12} className="ml-1 text-zinc-500" />}
       </button>
       {open && (