Fix fly-to vertical centering at non-1 zoom levels

Clicking ×N sometimes flew to a blank area far below the tree. Cause:
family-chart's cardToMiddle scales datum.x by the zoom factor k but not datum.y
(`y = height/2 - datum.y`, missing the ·k), so vertical centering is only
correct at k=1 and drifts by datum.y·(k−1) at any other zoom — worse the deeper
the person sits. That's why it worked only when the view happened to be near 1:1.

Compensate by pre-multiplying the y we pass to cardToMiddle by the current
scale, cancelling the library's missing ·k. x was already correct.

Signed-off-by: Justin Paul <justin@jpaul.me>

frontend/app/trees/[id]/tree/page.tsx

@@ -315,8 +315,12 @@ export default function TreePage() {
         try {
           const rect = svg.getBoundingClientRect();
           const scale = handlers.getCurrentZoom ? handlers.getCurrentZoom(svg).k : 1;
+          // family-chart's cardToMiddle scales datum.x by the zoom but NOT
+          // datum.y (a library bug), so vertical centering is only correct at
+          // scale 1 and drifts by datum.y·(k−1) otherwise — landing "below the
+          // tree". Pre-multiply y by the scale to cancel the missing ·k.
           handlers.cardToMiddle({
-            datum: xy,
+            datum: { x: xy.x, y: xy.y * scale },
             svg,
             svg_dim: { width: rect.width, height: rect.height },
             scale,