Stop the upcoming-lessons row collapsing onto itself

The lesson details and the actions rendered on top of each other. Three
things left the panel fragile, all fixed here.

The rules were bare class selectors while the visually identical
.us-slot row next to them was written as `#us-booking-app .us-slot`.
That inconsistency looks accidental, and it means a theme rule on
div/span/strong outranks the panel's layout and flattens it. Every
booking-page rule is now scoped under #us-booking-app.

The row's two columns were spans carrying display:flex, so the layout
only held while that declaration won. They are divs now — the layout no
longer depends on overriding the inline default.

The row had no flex-wrap and its title column no min-width:0, so a long
offering title could not shrink and shoved the status pill and Cancel
button out of the row. The 640px media query covered only the week
grid, leaving the busier lesson rows with no narrow-viewport handling at
all; they now stack details above actions.

BlockPreview mirrors the markup change so the editor preview matches.

Closes #133

assets/js/booking.js

@@ -582,16 +582,19 @@
     function lessonRowHtml(l) {
         const title    = l.offering_title ? escHtml(String(l.offering_title)) : 'Lesson';
         const duration = l.duration_minutes ? ` <span class="us-my-lesson-duration">(${escHtml(String(l.duration_minutes))} min)</span>` : '';
+        // The two columns are divs, not spans: as spans the layout only held up
+        // while the stylesheet's display:flex won, and a theme rule on span
+        // collapsed the row onto itself.
         return `
             <div class="us-my-lesson">
-                <span class="us-my-lesson-info">
+                <div class="us-my-lesson-info">
                     <strong class="us-my-lesson-title">${title}${duration}</strong>
                     <span class="us-my-lesson-when">${escHtml(dayLabel(dayKey(l.start_dt)))} · ${escHtml(timeOf(l.start_dt))}–${escHtml(timeOf(l.end_dt))}</span>
-                </span>
-                <span class="us-my-lesson-actions">
+                </div>
+                <div class="us-my-lesson-actions">
                     <span class="us-lesson-status us-lesson-status-${escHtml(String(l.status))}">${escHtml(lessonStatusLabel(String(l.status)))}</span>
                     <button type="button" class="us-cancel-lesson" data-lesson-id="${l.id}">Cancel</button>
-                </span>
+                </div>
             </div>`;
     }