Return to a bookable calendar after a booking is confirmed

showConfirmation() hid the slot list and put the confirmation in its place,
which is a dead end: a student wanting a second lesson had nothing to click
and no way back short of reloading the page. Enrolling in a group class did
the same thing.

The confirmation is now a dismissible notice above the calendar. The
calendar is reloaded first — so the slot just taken is already gone and the
upcoming-lessons panel is current — and the notice is shown over it, which
is why loadSlots() had to start returning its promise. "It worked" and "book
another" are the same screen.

The notice clears when dismissed, when another slot's form is opened, and on
any reload of the calendar. group-classes.js gets the identical treatment.

It is built from DOM nodes rather than innerHTML because the message can
carry a studio's e-transfer address, and it is toggled with the `hidden`
attribute rather than an inline display — an inline style would outrank the
stylesheet's display:flex and stack the notice's parts. `hidden` needs the
!important guard for the same reason the upcoming-lessons panel does: the
div{display:block} theme reset outranks the UA sheet.

The slotList/list `display = 'block'` lines went with it. Nothing hides
those any more, so restoring them each load only implied otherwise.

Verified in a headless browser against a stubbed REST API: booking twice in
a row without a reload, the booked slot leaving the calendar, the upcoming
panel updating, dismissal, the notice clearing when the next form opens, and
the notice staying hidden under div{display:block}.

Closes #143

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/features/lesson-booking.md

@@ -30,7 +30,8 @@ Students register for a private lesson by choosing an offering, picking a time (
 8. `POST /bookings` creates the lesson row(s) (`status = pending`), records answers and policy acceptances, marks `us_availability.is_booked = 1`, and links the payment. A booking with nothing owed (a free offering) creates no payment and is `confirmed` immediately.
 9. On successful payment (or comp) the lesson is `confirmed` and a receipt is emailed.
 10. Instructor sees the booking under **My Lessons** and may update status via `PATCH /bookings/{id}/status`.
-11. The booking page also shows the student their upcoming lessons (`GET /bookings`) — each with the booked offering's name and length, when it happens, a per-lesson status badge (pending payment / confirmed), and a **Cancel** button. Only the soonest five are shown; a **Show all** control reveals the rest. `GET /bookings` includes `offering_title` and `duration_minutes` for each lesson so the list needs no extra request.
+11. The confirmation is a **dismissible notice above the calendar**, not a screen of its own. The calendar is reloaded first — so the slot just taken is gone and the upcoming-lessons panel is current — and the notice is shown over it. Booking again therefore needs no page reload. The notice clears when it is dismissed, when another slot's booking form is opened, and on any reload of the calendar. `group-classes.js` does the same for enrolments.
+12. The booking page also shows the student their upcoming lessons (`GET /bookings`) — each with the booked offering's name and length, when it happens, a per-lesson status badge (pending payment / confirmed), and a **Cancel** button. Only the soonest five are shown; a **Show all** control reveals the rest. `GET /bookings` includes `offering_title` and `duration_minutes` for each lesson so the list needs no extra request.
 
 ## Lesson-Type Filter
 Not every open slot can be booked as every private-lesson type — a slot tied to