Tighten pipeline colour-bar spacing (#169)

The 1em margin under every lower pipeline colour bar kept the subtree
handle clear, but it also left a large gap between operators and their
outgoing edges. Remove that margin so edges sit close to the bars.

The lower colour bar already follows the collapsible property body in
the DOM. The body has zero height while properties are hidden and
expands in place when they are shown, so removing the margin is
sufficient. Hovering only reveals the absolutely positioned subtree
handle and does not resize the node.

Verified with the TypeScript build, the full ESLint check, and the
forkshare, materialized, unionall, and magicunnesting pipeline plans on
the development server.

Author: PerFuchs

query-graphs/src/ui/QueryNode.css

@@ -70,8 +70,6 @@
 }
 .qg-color-bar-below {
     margin-top: 4px;
-    /* Keep the bar above the subtree +/- handle at the bottom of the node. */
-    margin-bottom: 1em;
 }
 .qg-color-bar-seg {
     flex: 1 1 0;