feat(day): open any day from a calendar on the header's date

The written date in the day's header is now the way to any other day. A
click (or Enter, or Space) opens a month calendar beside it: the day on
screen is selected, every day that holds anything carries a dot read from
GET /api/day/index for the month shown, nothing past today can be picked,
and a Today button sits under the grid. Picking a day swaps it in like a
step, without a new history entry; Escape closes only the calendar and
focus returns to the date. The header keeps its one line and gains no
icon; the arrows stay. The same popover serves the docked column and both
sheets, with 40 px cells for a finger.

On /coach the day now docks beside the conversation like everywhere else,
right of the conversations, instead of opening as a sheet. A neighbouring
panel can fold the docked day to its edge without moving focus
(yieldDay).

The day's count line reads "3 values, 2 entries".

src/components/ui/date-field.tsx

@@ -315,7 +315,7 @@ function resolveOrder(
  * Map the app locale → the date-fns locale so the calendar's month / weekday
  * names match the UI language. Defaults to enUS for anything unmapped.
  */
-function resolveDateFnsLocale(
+export function resolveDateFnsLocale(
   locale: ReturnType<typeof useTranslations>["locale"],
 ): DateFnsLocale {
   switch (locale) {