Split availability windows into bookable lesson-length slots with weekly calendar views

Availability windows were stored and served as a single bookable row, so a
9:00 AM-4:00 PM window showed to students as one giant slot and booking it
consumed the whole day; past and multi-day windows also leaked into the
booking page as nonsense entries.

- Split windows into consecutive lesson-length slots on save (REST and admin
  form); each chunk is independently bookable and weekly recurrence creates a
  series per chunk so "reserve this time weekly" holds the same hour each week
- Reject windows spanning multiple days or shorter than the lesson length
  (400 invalid_window)
- Never return slots whose start has passed from GET /availability
- Migrate pre-split rows: Plugin::boot re-runs the Installer on version change
  and AvailabilityRepository::splitOversizedWindows() rewrites unbooked
  same-day oversized windows in place
- Display all times in 12-hour AM/PM form (booking page, wp-admin lists,
  editor previews)
- Add a List | Week view toggle to the student booking page and the
  instructor availability page, with previous/next-week navigation honouring
  the site's start_of_week option (new WeekCalendar helper)

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

assets/css/frontend.css

@@ -34,6 +34,83 @@
     margin-top: 8px;
 }
 
+.us-view-toggle {
+    display: flex;
+    gap: 8px;
+    margin-bottom: 12px;
+}
+
+.us-view-toggle button {
+    padding: 6px 16px;
+    border: 1px solid #ccc;
+    border-radius: 4px;
+    background: transparent;
+    cursor: pointer;
+}
+
+.us-view-toggle button.us-active {
+    background: #333;
+    border-color: #333;
+    color: #fff;
+}
+
+.us-week-nav {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    gap: 8px;
+    margin-bottom: 12px;
+}
+
+.us-week-nav button {
+    padding: 6px 12px;
+    border: 1px solid #ccc;
+    border-radius: 4px;
+    background: transparent;
+    cursor: pointer;
+}
+
+.us-week-grid {
+    display: grid;
+    grid-template-columns: repeat(7, 1fr);
+    gap: 8px;
+}
+
+.us-week-day {
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    padding: 8px;
+    min-height: 90px;
+}
+
+.us-week-day-heading {
+    margin: 0 0 8px;
+    font-size: 0.85em;
+    text-align: center;
+}
+
+.us-week-slot {
+    display: block;
+    width: 100%;
+    margin-bottom: 6px;
+}
+
+.us-week-empty {
+    display: block;
+    text-align: center;
+    opacity: 0.4;
+}
+
+@media (max-width: 640px) {
+    .us-week-grid {
+        grid-template-columns: 1fr;
+    }
+
+    .us-week-day {
+        min-height: 0;
+    }
+}
+
 /* Shown only in block-editor previews (see BlockPreview). */
 .us-editor-note {
     font-size: 0.85em;