Add a Sleep trend chart: cumulative sleep today vs yesterday and the recent average

Answers "is the puppy behind on sleep right now?": cumulative hours
slept sampled at each hour boundary, today's line ending at the current
moment, with yesterday and the mean of the last N days as reference
curves. N follows the 7/14/30 chart-days picker, and days with no sleep
logged are skipped so a young log doesn't drag the average down.

src/style.css

@@ -900,6 +900,33 @@ input.switch:checked::after { transform: translateX(18px); }
 .chart-svg .stl-hit { fill: transparent; cursor: pointer; }
 .chart-svg .stl-hit:hover { fill: var(--accent); fill-opacity: 0.08; }
 
+/* Sleep trend lines: today strongest, the reference curves lighter/dashed. */
+.chart-svg .trend-today {
+  stroke: var(--sleep);
+  stroke-width: 2.5;
+  stroke-linejoin: round;
+  fill: none;
+}
+.chart-svg .trend-yesterday {
+  stroke: var(--muted);
+  stroke-width: 1.5;
+  stroke-linejoin: round;
+  fill: none;
+  opacity: 0.75;
+}
+.chart-svg .trend-avg {
+  stroke: var(--sleep);
+  stroke-width: 1.5;
+  stroke-dasharray: 4 3;
+  stroke-linejoin: round;
+  fill: none;
+  opacity: 0.7;
+}
+.lg.trend-today .sw { background: var(--sleep); }
+.lg.trend-yesterday .sw { background: var(--muted); }
+.lg.trend-avg .sw { background: color-mix(in srgb, var(--sleep) 55%, var(--surface)); }
+.lg[hidden] { display: none; }
+
 .chart-svg .hm-cell { stroke: none; }
 .chart-svg .hm-pee { fill: var(--pee); }
 .chart-svg .hm-poo { fill: var(--poo); }