Drop the training grid's caption

Third of the explanatory lines to go, and the same case as the other two: it
told you darker meant more, which a heatmap says by being one.

The tap-to-open-that-day half went with it, but the behaviour did not — the
cells are still wired through setChartSVG. Losing the sentence costs the only
hint that they are tappable at all, which is the trade the other captions made
too.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-09-01", "text": "Dropped the “Darker = more sessions that day” caption under the training consistency grid. Tapping a cell still opens that day" },
   { "date": "2026-09-01", "text": "Dropped two hint lines: the “Based on N pee gaps…” sentence under the Timing charts, and the “Rule of thumb at this age…” one in the Walks panel. The charts above them already say it. The age-based walking goal is still there, drawn as the goal line on the Walk trend" },
   { "date": "2026-09-01", "text": "On the dark theme the awake timers are a warm near-white now — both “Awake for” at the top and the pill in the day bar — instead of yellow text on a yellow-tinted pill. The light theme keeps its dark gold, where white would disappear into a near-white pill" },
   { "date": "2026-09-01", "text": "Toned the awake timer down: the pill in the day bar is a fainter yellow and its text a good deal darker, so “Awake for” is comfortable to read rather than technically legible. The Awake labels elsewhere darkened with it" },