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]>

assets/css/frontend.css

@@ -549,6 +549,49 @@
     color: #1a7d2e;
 }
 
+/*
+ * The "you're booked" / "you're enrolled" notice. It sits above the calendar
+ * or class list rather than replacing it, so it needs to read as a banner
+ * about something that just happened — not as the page's content.
+ */
+/*
+ * `[hidden]` is a UA-stylesheet rule, so the widespread `div { display: block }`
+ * theme reset outranks it — the same trap the upcoming-lessons panel hit. An
+ * author !important is the only way to win, and it has to sit before the
+ * display rule it guards against.
+ */
+.us-notice[hidden] {
+    display: none !important;
+}
+
+.us-notice {
+    display: flex;
+    flex-wrap: wrap;
+    justify-content: space-between;
+    align-items: center;
+    gap: 8px 16px;
+    margin-bottom: 16px;
+    padding: 12px 16px;
+    border: 1px solid #b7dfc0;
+    border-left-width: 4px;
+    border-radius: 4px;
+    background: #f2faf4;
+    color: #1a5c2a;
+}
+
+.us-notice p {
+    margin: 0;
+}
+
+.us-notice-dismiss {
+    background: transparent;
+    border: 1px solid currentColor;
+    border-radius: 4px;
+    padding: 4px 12px;
+    color: inherit;
+    cursor: pointer;
+}
+
 /* Shown only in block-editor previews (see BlockPreview). */
 .us-editor-note {
     font-size: 0.85em;