feat: preserve messages after compaction (IndexedDB cache) + show agent name in header

- Add IndexedDB message cache to retain pre-compaction history locally
- Show compaction separator with amber styling when messages are compacted
- Archived messages render at 60% opacity above the separator
- Display agent name (from gateway identity) in header instead of 'PinchChat' when available
- Fallback to 'PinchChat' when no agent name is configured
- Add i18n keys for compaction separator (EN/FR)

Closes #72, #69

src/lib/i18n.ts

@@ -147,6 +147,7 @@ const en = {
   'message.bookmark': 'Bookmark message',
   'message.removeBookmark': 'Remove bookmark',
   'chat.bookmarks': 'Bookmarks',
+  'chat.contextCompacted': 'Context compacted — older messages cached locally',
 } as const;
 
 const fr: Record<keyof typeof en, string> = {
@@ -273,6 +274,7 @@ const fr: Record<keyof typeof en, string> = {
   'message.bookmark': 'Marquer le message',
   'message.removeBookmark': 'Retirer le marque-page',
   'chat.bookmarks': 'Marque-pages',
+  'chat.contextCompacted': 'Contexte compacté — anciens messages en cache local',
 };
 
 export type TranslationKey = keyof typeof en;