Replace the date picker with a month grid of our own

The day bar had run out of room: two timers and four day controls came to more
than the bar's width on every phone, so it wrapped onto two rows whenever a walk
was running. Shaving pixels off both groups was not enough — even the most
compact form of it only fitted a 428px iPhone Plus.

The way out was structural. The browser's date picker is a sheet that covers
the screen, which is backwards here: the reason to change day is to see what
the figures did on it, and a modal hides exactly the thing you opened it for.
So the date button now opens a small panel under the bar instead — month name
with arrows, locale-ordered weekday initials, six rows of days — with the
overview still on screen and updating as you move through it.

That also solved the width, because Today belongs inside the grid rather than
beside it. The day controls drop from about 204px to 111px, which is enough for
both timers on one row from 320px up; only a 280px foldable cover still wraps.
The bar keeps its wrapping for that case and for large system font sizes.

The hidden input stays as the value everything reads — selectedDay() and every
caller are untouched, and only the input's own picker is no longer opened.

Six rows always, so the panel does not change height from month to month.
Future days are disabled, matching the bar's → being disabled on today. The
week starts where Intl says it does for the reader's locale, falling back to
Monday. Arrow keys walk the grid and pull the neighbouring month into view at
the edges.

The grid arithmetic is checked rather than eyeballed, both week starts across
every month of a year, leap years and year boundaries — including February
2026, which begins on a Sunday and so needs six leading days from January under
Monday weeks. That is the case a naive `1 - getDay()` gets wrong, and it would
have been invisible until somebody happened to open that month.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-09-08", "text": "The date now opens a small month grid of the app's own instead of the browser's date picker. The browser's one is a sheet that covers the screen, which is backwards when the reason to change day is to see what the numbers did on it — this one sits under the bar with the overview still visible and updating as you move. ← and → still step a day at a time; the grid is for jumping further, and the Today button now lives inside it. That is what made room for the walk timer and the sleep timer to sit on one row: the top bar no longer splits onto two rows on a phone, except on the very smallest. The two timers count in minutes now rather than seconds — the big card on Today still ticks in seconds, which is where you look if you want them" },
   { "date": "2026-09-08", "text": "A walk in progress now has a timer in the frozen bar at the top, next to the asleep/awake one, counting from when the walk started — so you can see how long you have been out from any tab without going to look. Tapping it ends the walk, the same way tapping the sleep timer logs the sleep boundary. On a narrow phone two timers no longer fit beside the date controls, so the bar splits onto two rows while a walk is on and goes back to one when it ends. The big timer card on Today shows the walk too while there is one — you are awake on a walk either way, so the walk is the more useful of the two" },
   { "date": "2026-09-07", "text": "Going back to the Today tab no longer jumps the viewport either — by tapping it or with the back button. The other tabs stopped jumping in the last change, but returning to Today is a history step, and the browser was restoring the scroll position from when you last left it" },
   { "date": "2026-09-07", "text": "The tab row now matches the frozen row above it that holds the date. It was a different shade, and the two sat as separate bars with the date row's rounded corners cutting between them; they share the same card colour now, and once you have scrolled the log buttons away they join into a single rounded block instead of two stacked ones" },