feat: add message metadata viewer on hover

Small info button appears on hover of each message bubble.
Click to expand a panel showing raw message metadata (id, role,
timestamp, channel, sender info, etc.) from the gateway.
Useful for debugging and understanding message routing.
Collapsed by default, doesn't clutter the UI.

Closes feedback item #39

src/lib/i18n.ts

@@ -73,6 +73,7 @@ const en = {
   'message.copy': 'Copy message',
   'message.copied': 'Copied!',
   'message.retry': 'Resend message',
+  'message.metadata': 'Message details',
 
   // Timestamps
   'time.yesterday': 'Yesterday',
@@ -160,6 +161,7 @@ const fr: Record<keyof typeof en, string> = {
   'message.copy': 'Copier le message',
   'message.copied': 'Copié !',
   'message.retry': 'Renvoyer le message',
+  'message.metadata': 'Détails du message',
 
   'time.yesterday': 'Hier',
   'time.today': "Aujourd'hui",