fix(06.2): shared badge and recipient handle missing in FileManagerView

- shares.py grant_share: include recipient_handle in response so ShareModal shows the name immediately without reload
- FileManagerView: add Shared pill badge next to document name (badge only existed in DocumentCard, not the main file manager view)
- FileManagerView ShareModal: wire @unshared to clear is_shared flag when last recipient is removed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/shares.py

@@ -152,6 +152,7 @@ async def grant_share(
         "document_id": str(share.document_id),
         "owner_id": str(share.owner_id),
         "recipient_id": str(share.recipient_id),
+        "recipient_handle": recipient.handle,
         "permission": share.permission,
         "created_at": share.created_at.isoformat() if share.created_at else None,
     }