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/lib/i18n.ts

@@ -49,6 +49,10 @@ const en = {
   // Tool call
   'tool.result': 'Result',
 
+  // Connection banner
+  'connection.reconnecting': 'Connection lost — reconnecting…',
+  'connection.reconnected': 'Reconnected!',
+
   // Timestamps
   'time.yesterday': 'Yesterday',
 } as const;
@@ -89,6 +93,9 @@ const fr: Record<keyof typeof en, string> = {
 
   'tool.result': 'Résultat',
 
+  'connection.reconnecting': 'Connexion perdue — reconnexion…',
+  'connection.reconnected': 'Reconnecté !',
+
   'time.yesterday': 'Hier',
 };