feat: live markdown preview toggle in chat input

- Eye icon button next to file picker toggles preview on/off
- Shows rendered markdown above textarea in real-time
- Lazy-loads ReactMarkdown (no bundle impact when off)
- Preference persisted in localStorage
- i18n: EN/FR labels for show/hide preview

src/lib/i18n.ts

@@ -40,6 +40,8 @@ const en = {
   'chat.attachFile': 'Attach file',
   'chat.send': 'Send',
   'chat.stop': 'Stop',
+  'chat.showPreview': 'Preview markdown',
+  'chat.hidePreview': 'Hide preview',
   'chat.scrollToBottom': 'New messages',
   'chat.collapseTools': 'Collapse all tools',
   'chat.expandTools': 'Expand all tools',
@@ -151,6 +153,8 @@ const fr: Record<keyof typeof en, string> = {
   'chat.attachFile': 'Joindre un fichier',
   'chat.send': 'Envoyer',
   'chat.stop': 'Arrêter',
+  'chat.showPreview': 'Aperçu markdown',
+  'chat.hidePreview': 'Masquer l\'aperçu',
   'chat.scrollToBottom': 'Nouveaux messages',
   'chat.collapseTools': 'Replier tous les outils',
   'chat.expandTools': 'Déplier tous les outils',