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/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-26", "text": "The pedigree is now zoomable — use the +/− buttons, ⌘/Ctrl-scroll, or pinch on a phone — to fit a wide tree on screen or zoom in for detail; your zoom level is remembered" },
   { "date": "2026-07-26", "text": "In the pedigree, a dog that fills more than one spot (pedigree collapse, common in a breed's older lines) now carries a ×N badge — tap it to highlight every place that dog appears in the tree" },
   { "date": "2026-07-26", "text": "The pedigree now reads top-down like a family tree — your dog on top with its sire and dam branching below — showing three generations at a glance, with each dog expandable to trace the line further back" },
   { "date": "2026-07-26", "text": "The pedigree ID set in Settings now syncs reliably to your other devices — it's no longer dropped when two devices' clocks disagree" },