feat: improve accessibility — semantic HTML landmarks, skip-to-content link, ARIA attributes

- Replace div with <main> for primary chat pane (screen reader landmark)
- Replace div with <section> for split pane
- Add skip-to-content link for keyboard navigation (Tab → jump to chat input)
- Add aria-expanded and aria-label to tool call badge buttons
- Add id='chat-input' to textarea for skip link target
- Add i18n keys for new ARIA labels (EN + FR)

src/lib/i18n.ts

@@ -63,6 +63,9 @@ const en = {
   'sidebar.close': 'Close sidebar',
   'sidebar.clearSearch': 'Clear search',
   'split.close': 'Close split view',
+  'app.mainChat': 'Main chat',
+  'app.splitPane': 'Split pane',
+  'app.skipToChat': 'Skip to chat input',
 
   // Thinking
   'thinking.label': 'Thinking',
@@ -180,6 +183,9 @@ const fr: Record<keyof typeof en, string> = {
   'sidebar.close': 'Fermer la barre latérale',
   'sidebar.clearSearch': 'Effacer la recherche',
   'split.close': 'Fermer la vue scindée',
+  'app.mainChat': 'Chat principal',
+  'app.splitPane': 'Volet scindé',
+  'app.skipToChat': 'Aller au champ de saisie',
 
   'thinking.label': 'Réflexion',
   'thinking.reasoning': 'Réflexion…',