Project today's end-of-day sleep total on the Sleep trend chart

A dashed tail continues today's line from now to midnight by adding the
increments the N-day average curve makes over the same stretch, so the
projection follows the usual daily rhythm instead of extrapolating the
current rate (which overshoots right after a long night). The legend
shows the projected total; with no history there's no average and no
projection.

src/app.js

@@ -1318,7 +1318,27 @@
       }
     }
 
-    return { today, yesterday, avg, avgDays };
+    // Where today is likely to end up: continue today's line by adding what
+    // 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.
+    let projected = null;
+    if (avg) {
+      const nowPt = today[today.length - 1];
+      const avgAt = (x) => {
+        const lo = Math.floor(x);
+        const hi = Math.min(24, lo + 1);
+        return avg[lo].y + (avg[hi].y - avg[lo].y) * (x - lo);
+      };
+      projected = [{ x: nowPt.x, y: nowPt.y }];
+      for (let h = Math.ceil(nowPt.x); h <= 24; h++) {
+        if (h <= nowPt.x) continue; // now landing exactly on an hour boundary
+        projected.push({ x: h, y: nowPt.y + avgAt(h) - avgAt(nowPt.x) });
+      }
+    }
+
+    return { today, yesterday, avg, avgDays, projected };
   }
 
   function drawSleepTrendChart(curves) {
@@ -1330,9 +1350,10 @@
     const innerH = H - MT - MB;
 
     const series = [
-      // Reference lines first so today draws on top of them.
+      // 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.projected, cls: "trend-projected", label: "Projected end of day" },
       { pts: curves.today,     cls: "trend-today",     label: "Today" },
     ].filter(s => s.pts && s.pts.length > 1);
 
@@ -1377,9 +1398,15 @@
     const yLegend = document.getElementById("legend-trend-yesterday");
     const aLegend = document.getElementById("legend-trend-avg");
     const aText   = document.getElementById("legend-trend-avg-text");
+    const pLegend = document.getElementById("legend-trend-projected");
+    const pText   = document.getElementById("legend-trend-projected-text");
     if (yLegend) yLegend.hidden = !curves.yesterday;
     if (aLegend) aLegend.hidden = !curves.avg;
     if (aText)   aText.textContent = `${curves.avgDays}-day avg`;
+    if (pLegend) pLegend.hidden = !curves.projected;
+    if (pText && curves.projected) {
+      pText.textContent = `Projected ~${curves.projected[curves.projected.length - 1].y.toFixed(1)}h`;
+    }
   }
 
   // ---------- weight ----------