Label every 1h gridline on both sleep charts

Instead of thinning labels on dense axes, keep them all and drop the
label font to 7px past 12 steps (bars) / 16 steps (trend).

src/style.css

@@ -597,6 +597,8 @@ dialog menu {
 }
 .chart-svg { display: block; width: 100%; height: auto; }
 .chart-svg text { fill: var(--muted); font-size: 9px; }
+/* Dense 1h y-axes: smaller labels so every gridline can carry one. */
+.chart-svg text.y-dense { font-size: 7px; }
 .chart-svg .grid {
   stroke: var(--border);
   stroke-dasharray: 2 3;