feat: add scroll-to-bottom button when scrolled up in chat

Floating button appears when user scrolls away from the bottom of the
conversation, making it easy to jump back to the latest messages.
Includes i18n labels (EN/FR) and smooth scroll animation.

src/lib/i18n.ts

@@ -36,6 +36,7 @@ const en = {
   'chat.attachFile': 'Attach file',
   'chat.send': 'Send',
   'chat.stop': 'Stop',
+  'chat.scrollToBottom': 'New messages',
   'chat.messages': 'Chat messages',
 
   // Sidebar
@@ -78,6 +79,7 @@ const fr: Record<keyof typeof en, string> = {
   'chat.attachFile': 'Joindre un fichier',
   'chat.send': 'Envoyer',
   'chat.stop': 'Arrêter',
+  'chat.scrollToBottom': 'Nouveaux messages',
   'chat.messages': 'Messages du chat',
 
   'sidebar.title': 'Sessions',