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/types/index.ts

@@ -14,6 +14,10 @@ export interface ChatMessage {
   streamStartedAt?: number;
   /** Total generation time in milliseconds (set when streaming ends) */
   generationTimeMs?: number;
+  /** True if this message was restored from local cache (pre-compaction) */
+  isArchived?: boolean;
+  /** True if this is a visual separator showing where compaction occurred */
+  isCompactionSeparator?: boolean;
 }
 
 export type MessageBlock =