fix: correct TYPE_ID separator and version in TypeScript constants

- TYPE_ID: 'cxdb:ConversationItem' (colon) → 'cxdb.ConversationItem' (dot)
  to match the registry bundle which has always used dot separators
- TYPE_VERSION: 2 → 3 to match the current registry schema version

The colon separator was invalid in contexts that parse the type ID as a
namespace path. The version mismatch caused the frontend to miss schema
features registered at version 3.

Author: colombod

frontend/types/conversation.ts

@@ -11,9 +11,9 @@
  * Legacy types (assistant, tool_call, tool_result) are kept for backward compatibility.
  */
 
-// Type ID constants matching the Go package
-export const TYPE_ID_CONVERSATION_ITEM = 'cxdb:ConversationItem';
-export const TYPE_VERSION_CONVERSATION_ITEM = 2;
+// Type ID constants matching the registry bundle
+export const TYPE_ID_CONVERSATION_ITEM = 'cxdb.ConversationItem';
+export const TYPE_VERSION_CONVERSATION_ITEM = 3;
 
 // =============================================================================
 // Enums