Make y-axis of sleep graph more granular

src/app.js

@@ -684,6 +684,14 @@
     return { yMax: m, steps: 5 };
   }
 
+  // Sleep-specific axis: always 2-hour granularity, capped at 24h/day,
+  // for a more readable picture of typical 10–18 h puppy sleep.
+  function niceAxisSleepHours(rawMax) {
+    if (!(rawMax > 0)) return { yMax: 2, steps: 2 };
+    const m = Math.min(24, Math.max(2, Math.ceil(rawMax / 2) * 2));
+    return { yMax: m, steps: m / 2 };
+  }
+
   function escapeText(s) {
     return String(s).replace(/[&<>"']/g, c => (
       { "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;", "'": "&#39;" }[c]
@@ -710,7 +718,7 @@
     const innerH = H - MT - MB;
 
     const rawMax = Math.max(...days.map(d => d.sleepHours));
-    const { yMax, steps: ySteps } = niceAxis(rawMax);
+    const { yMax, steps: ySteps } = niceAxisSleepHours(rawMax);
 
     const gap = 6;
     const barW = (innerW - (days.length - 1) * gap) / days.length;