fix(page): gate sync when snapshot has unknown block IDs

rollbackSyncedPage previously guarded only on snapshot.Truncated. If a
page contains blocks that cannot be represented in markdown, the
PageMarkdown snapshot carries their IDs in UnknownBlockIDs, and a
replace_content rollback would replay a lossy markdown body and
permanently drop those blocks. Treat non-empty UnknownBlockIDs as an
unsafe rollback source and fail early at the pre-sync gate with the
same pattern used for truncation, so page sync does not silently lose
content on recovery.

Author: 0xble

cmd/page.go

@@ -604,6 +604,11 @@ func runPageSync(ctx *Context, file, title, parent, parentDB, icon string, skipL
 				output.PrintError(finalErr)
 				return finalErr
 			}
+			if len(snapshot.UnknownBlockIDs) > 0 {
+				finalErr := fmt.Errorf("cannot sync local images safely: page %s contains %d block(s) that cannot be represented in markdown, so rollback on a failed substitution would drop them. Retry without local images or remove the unsupported blocks before syncing", fm.NotionID, len(snapshot.UnknownBlockIDs))
+				output.PrintError(finalErr)
+				return finalErr
+			}
 		}
 
 		body, localUploads, err = prepareLocalImageUploads(ctx, bgCtx, file, body)

cmd/page_local_images.go

@@ -232,6 +232,9 @@ func rollbackSyncedPage(ctx context.Context, client pageUpdater, pageID string,
 	if snapshot.Truncated {
 		return fmt.Errorf("skipped rollback: page markdown snapshot was truncated; replaying would lose content")
 	}
+	if len(snapshot.UnknownBlockIDs) > 0 {
+		return fmt.Errorf("skipped rollback: page markdown snapshot omits %d block(s) that cannot be represented in markdown; replaying would drop them", len(snapshot.UnknownBlockIDs))
+	}
 	return client.UpdatePage(ctx, mcp.UpdatePageRequest{
 		PageID:     pageID,
 		Command:    "replace_content",

cmd/page_local_images_test.go

@@ -199,6 +199,21 @@ func TestRollbackSyncedPageSkipsTruncatedSnapshot(t *testing.T) {
 	}
 }
 
+func TestRollbackSyncedPageSkipsUnknownBlocks(t *testing.T) {
+	snapshot := &api.PageMarkdown{
+		Markdown:        "# Title\n\nLossy content\n",
+		UnknownBlockIDs: []string{"block-1", "block-2"},
+	}
+
+	err := rollbackSyncedPage(context.Background(), nil, "page-id", snapshot)
+	if err == nil {
+		t.Fatalf("rollbackSyncedPage returned nil error; expected unknown-blocks error")
+	}
+	if !strings.Contains(err.Error(), "cannot be represented in markdown") {
+		t.Fatalf("rollbackSyncedPage error = %q, want it to mention unrepresentable blocks", err.Error())
+	}
+}
+
 func TestRollbackSyncedPageSkipsNilSnapshot(t *testing.T) {
 	if err := rollbackSyncedPage(context.Background(), nil, "page-id", nil); err != nil {
 		t.Fatalf("rollbackSyncedPage returned %v, want nil", err)