Log training sessions onto the selected day, not always today

The Log button stamps the selected day at the current time of day, so a
forgotten session can be back-filled from that day's view. The snackbar
says which day it landed on when it isn't today; Undo/Add note still
work on it.

src/app.js

@@ -1711,8 +1711,19 @@
       logBtn.textContent = "Log";
       logBtn.addEventListener("click", (e) => {
         e.stopPropagation();
-        const ev = addEvent("training", "", Date.now(), { exerciseId: ex.id });
-        showSnackbar(`${ex.name} logged`, ev);
+        // Log onto the selected day (at the current time of day), so a session
+        // that was forgotten yesterday can be back-filled from that day's view.
+        const day = selectedDay();
+        const onToday = ymd(day) === ymd(new Date());
+        let at = Date.now();
+        if (!onToday) {
+          const now = new Date();
+          at = new Date(day.getFullYear(), day.getMonth(), day.getDate(),
+                        now.getHours(), now.getMinutes(), now.getSeconds()).getTime();
+        }
+        const ev = addEvent("training", "", at, { exerciseId: ex.id });
+        const dayStr = day.toLocaleDateString(undefined, { month: "short", day: "numeric" });
+        showSnackbar(onToday ? `${ex.name} logged` : `${ex.name} logged for ${dayStr}`, ev);
       });
 
       row.appendChild(main);