Make the Sleep trend chart follow the selected day

The curves were always computed relative to today regardless of the day
picker. Anchor them to selectedDay() instead: a past day shows its full
24h curve against the day before it and the average of the days leading
up to it, with no now-cap and no projection. Legend and tooltip labels
show the actual dates when viewing a past day.

src/app.js

@@ -1278,9 +1278,10 @@
   }
 
   // ---------- sleep trend ----------
-  // Cumulative hours slept as the day progresses: today's running total (up to
-  // now) against yesterday's full curve and the mean of the last N full days,
-  // where N follows the 7/14/30 chart-days picker.
+  // Cumulative hours slept as the selected day progresses: that day's running
+  // total (up to now when it's today, else the full 24h) against the previous
+  // day's full curve and the mean of the N full days before it, where N
+  // follows the 7/14/30 chart-days picker.
   // Each curve is a list of { x: hour-of-day 0..24, y: cumulative hours }.
   function sleepTrendCurves(events) {
     const HOUR = 3_600_000;
@@ -1294,8 +1295,10 @@
       }
       return total;
     };
+    const day = selectedDay();
+    const isToday = ymd(day) === ymd(new Date());
     const dayStartTs = (daysAgo) => {
-      const d = startOfDay(new Date());
+      const d = startOfDay(day);
       d.setDate(d.getDate() - daysAgo);
       return d.getTime();
     };
@@ -1314,7 +1317,8 @@
       return pts;
     };
 
-    const today = curveFor(dayStartTs(0), Date.now());
+    // A past day is complete, so its curve runs the full 24h uncapped.
+    const today = curveFor(dayStartTs(0), isToday ? Date.now() : null);
 
     const yesterdayCurve = curveFor(dayStartTs(1));
     const yesterday = yesterdayCurve[24].y > 0 ? yesterdayCurve : null;
@@ -1339,9 +1343,10 @@
     // the average day typically adds between now and midnight. That respects
     // the time-of-day rhythm (night sleep, nap clusters), unlike a linear
     // rate extrapolation, which overshoots wildly just after a long night.
-    // No history → no average → no projection.
+    // No history → no average → no projection. Past days are already complete,
+    // so there is nothing to project.
     let projected = null;
-    if (avg) {
+    if (avg && isToday) {
       const nowPt = today[today.length - 1];
       const avgAt = (x) => {
         const lo = Math.floor(x);
@@ -1355,7 +1360,13 @@
       }
     }
 
-    return { today, yesterday, avg, avgDays, projected };
+    // Labels follow the selection so a past day reads as its date, not "Today".
+    const fmtDay = (daysAgo) =>
+      new Date(dayStartTs(daysAgo)).toLocaleDateString(undefined, { month: "short", day: "numeric" });
+    const dayLabel = isToday ? "Today" : fmtDay(0);
+    const prevDayLabel = isToday ? "Yesterday" : fmtDay(1);
+
+    return { today, yesterday, avg, avgDays, projected, dayLabel, prevDayLabel };
   }
 
   function drawSleepTrendChart(curves, target) {
@@ -1369,9 +1380,9 @@
     const series = [
       // Reference lines first so today (and its projected tail) draw on top.
       { pts: curves.avg,       cls: "trend-avg",       label: `${curves.avgDays}-day average` },
-      { pts: curves.yesterday, cls: "trend-yesterday", label: "Yesterday" },
+      { pts: curves.yesterday, cls: "trend-yesterday", label: curves.prevDayLabel },
       { pts: curves.projected, cls: "trend-projected", label: "Projected end of day" },
-      { pts: curves.today,     cls: "trend-today",     label: "Today" },
+      { pts: curves.today,     cls: "trend-today",     label: curves.dayLabel },
     ].filter(s => s.pts && s.pts.length > 1);
 
     // The axis must reach the goal band's top even when the data doesn't yet.
@@ -1452,11 +1463,11 @@
     // write each curve's slept-hours total into its chip.
     const chip = (id) => document.getElementById(id);
     const hrs = (pts) => `${pts[pts.length - 1].y.toFixed(1)}h`;
-    chip("legend-trend-today-text").textContent = `Today ${hrs(curves.today)}`;
+    chip("legend-trend-today-text").textContent = `${curves.dayLabel} ${hrs(curves.today)}`;
     const yLegend = chip("legend-trend-yesterday");
     yLegend.hidden = !curves.yesterday;
     if (curves.yesterday) {
-      chip("legend-trend-yesterday-text").textContent = `Yesterday ${hrs(curves.yesterday)}`;
+      chip("legend-trend-yesterday-text").textContent = `${curves.prevDayLabel} ${hrs(curves.yesterday)}`;
     }
     const aLegend = chip("legend-trend-avg");
     aLegend.hidden = !curves.avg;