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/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-15", "text": "Logging a training session while viewing another day puts it on that day (the snackbar tells you where it went)" },
   { "date": "2026-07-15", "text": "The Sleep trend yesterday line is orange instead of gray, which was hard to see" },
   { "date": "2026-07-15", "text": "The Sleep trend average line is teal now, so it doesn't blend in with today's blue line and its projection" },
   { "date": "2026-07-15", "text": "Tapping a day in a chart selects it without jumping down to the history" },