Make the pedigree tree zoomable

Add zoom to the pedigree: −/reset/+ buttons in the header, ctrl/⌘ + wheel,
and two-finger pinch. Scaling uses the CSS `zoom` property (not transform)
so the container reflows and every part stays reachable by scrolling at any
level. Clamped to 40–160% and the level is remembered per device.

src/style.css

@@ -1066,8 +1066,25 @@ section.collapsed > :not(h2) { display: none; }
   padding: 12px 4px 8px;
 }
 .pedigree-header h1 { font-size: 1.4rem; }
+.ped-zoom {
+  margin-left: auto;
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  flex-shrink: 0;
+}
+.ped-zoom .icon-btn { padding: 4px 9px; font-size: 1.1rem; }
+#ped-zoom-reset {
+  padding: 5px 8px;
+  font-size: 0.8rem;
+  font-variant-numeric: tabular-nums;
+  min-width: 48px;
+}
 
 .pedigree-hint { margin: 4px 0 12px; }
+/* The tree scales with the CSS `zoom` property, which reflows so the container
+   still scrolls to reach the edges at any zoom. */
+.pedigree-tree { touch-action: pan-x pan-y; }
 
 .pedigree-status {
   margin: 10px 0;