feat: connection lost/reconnected banner with i18n

Adds a slim animated banner below the header that:
- Shows 'Connection lost — reconnecting…' with spinner when WS drops
- Flashes 'Reconnected!' with auto-dismiss after 3s on recovery
- Only appears after initial connection (not on first load)
- Supports EN/FR via i18n system
- Uses aria role=alert for accessibility

src/App.tsx

@@ -4,6 +4,7 @@ import { Header } from './components/Header';
 import { Sidebar } from './components/Sidebar';
 import { Chat } from './components/Chat';
 import { LoginScreen } from './components/LoginScreen';
+import { ConnectionBanner } from './components/ConnectionBanner';
 
 export default function App() {
   const {
@@ -38,6 +39,7 @@ export default function App() {
       />
       <div className="flex-1 flex flex-col min-w-0">
         <Header status={status} sessionKey={activeSession} onToggleSidebar={() => setSidebarOpen(!sidebarOpen)} activeSessionData={sessions.find(s => s.key === activeSession)} onLogout={logout} />
+        <ConnectionBanner status={status} />
         <Chat messages={messages} isGenerating={isGenerating} status={status} onSend={sendMessage} onAbort={abort} />
       </div>
     </div>