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/index.html

@@ -280,6 +280,11 @@
       <header class="pedigree-header">
         <button type="button" id="pedigree-back" class="ghost icon-btn" aria-label="Back to tracker" title="Back">←</button>
         <h1>🌳 Pedigree</h1>
+        <div class="ped-zoom" role="group" aria-label="Zoom">
+          <button type="button" id="ped-zoom-out" class="ghost icon-btn" aria-label="Zoom out" title="Zoom out">−</button>
+          <button type="button" id="ped-zoom-reset" class="ghost" aria-label="Reset zoom" title="Reset zoom">100%</button>
+          <button type="button" id="ped-zoom-in" class="ghost icon-btn" aria-label="Zoom in" title="Zoom in">+</button>
+        </div>
       </header>
       <main class="pedigree-main">
         <p class="muted-note pedigree-hint">