Increase the daily counts chart y-axis granularity

1-unit gridlines up to 10, 2-unit up to 20, 5-unit beyond — previously
anything above 10 was squeezed into five segments.

src/app.js

@@ -838,19 +838,20 @@
   }
 
   // Pick a chart Y maximum and tick count so every tick label is a clean
-  // whole number (avoids 0, 0, 1, 1, 2 from rounding fractional steps).
+  // whole number (avoids 0, 0, 1, 1, 2 from rounding fractional steps):
+  // 1-unit gridlines up to 10, 2-unit up to 20, 5-unit beyond.
   function niceAxis(rawMax) {
     if (!(rawMax > 0)) return { yMax: 1, steps: 1 };
-    if (rawMax <= 4) {
+    if (rawMax <= 10) {
       const m = Math.ceil(rawMax);
       return { yMax: m, steps: m };
     }
-    if (rawMax <= 10) {
+    if (rawMax <= 20) {
       const m = Math.ceil(rawMax / 2) * 2;
       return { yMax: m, steps: m / 2 };
     }
     const m = Math.ceil(rawMax / 5) * 5;
-    return { yMax: m, steps: 5 };
+    return { yMax: m, steps: m / 5 };
   }
 
   // Sleep-specific axis: always 2-hour granularity, capped at 24h/day,