feat: add elapsed time counter to thinking indicator

Shows how long the agent has been thinking (e.g. '5s', '1m 23s').
Timer appears after 2 seconds to avoid flicker on fast responses.
Helps users gauge if a request is still processing or stuck.

src/lib/i18n.ts

@@ -42,6 +42,7 @@ const en = {
   'chat.stop': 'Stop',
   'chat.scrollToBottom': 'New messages',
   'chat.messages': 'Chat messages',
+  'chat.thinking': 'Thinking…',
 
   // Sidebar
   'sidebar.title': 'Sessions',
@@ -121,6 +122,7 @@ const fr: Record<keyof typeof en, string> = {
   'chat.stop': 'Arrêter',
   'chat.scrollToBottom': 'Nouveaux messages',
   'chat.messages': 'Messages du chat',
+  'chat.thinking': 'Réflexion…',
 
   'sidebar.title': 'Sessions',
   'sidebar.empty': 'Aucune session',