fix: proper WCAG heading structure for day sections

Wrap the disclosure button in <h2> per the WAI-ARIA accordion pattern.
Screen readers now announce a heading level 2 for each day and users
can navigate between days with the H key. The button's accessible name
comes from its text content (date + meta), not a redundant aria-label.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web.py

@@ -690,6 +690,7 @@ button.cut:hover:not(:disabled){background:#1e3a8a}
 button.day-hl{color:var(--green);border-color:#166534;background:#052e16;font-size:11px}
 button.day-hl:hover:not(:disabled){background:#0a3d1f}
 button.day-hl:disabled{opacity:.5;cursor:default}
+h2.day-heading{margin:0;font-size:inherit;font-weight:inherit;line-height:inherit;flex:1 1 auto}
 .day-hl-container{background:var(--bg);border:1px solid var(--brd);border-top:none;padding:8px 12px 12px}
 table.day-table{width:100%;border-collapse:collapse;border:1px solid var(--brd);border-top:none}
 svg.day-timeline{display:block;width:100%;height:22px}
@@ -1112,14 +1113,15 @@ function renderFiles(files) {
     headBar.className = 'day-heading-bar' + (expanded ? ' open' : '');
     headBar.id = 'dayhead-' + dayId;
     headBar.innerHTML = `
+      <h2 class="day-heading">
         <button class="day-toggle" id="daytgl-${dayId}"
           aria-expanded="${expanded}"
-        aria-controls="daytbl-${dayId}"
-        aria-label="${expanded ? 'Collapse' : 'Expand'} ${esc(day)}">
+          aria-controls="daytbl-${dayId}">
           <span class="day-arrow" aria-hidden="true">${expanded ? '▼' : '▶'}</span>
-        <strong>${esc(day)}</strong>
+          ${esc(day)}
           <span class="day-meta">${fileStr}${durStr}${sizeStr}</span>
         </button>
+      </h2>
       ${canHl ? `<button class="day-hl" id="dayhln-${dayId}"
         aria-label="Show day highlights for ${esc(day)}">★ Highlights</button>` : ''}`;
     section.appendChild(headBar);
@@ -1227,7 +1229,6 @@ function renderFiles(files) {
       dayExpanded.set(dayId, nowExp);
       const tgl = document.getElementById('daytgl-' + dayId);
       tgl.setAttribute('aria-expanded', nowExp);
-      tgl.setAttribute('aria-label', (nowExp ? 'Collapse' : 'Expand') + ' ' + day);
       tgl.querySelector('.day-arrow').textContent = nowExp ? '▼' : '▶';
       headBar.classList.toggle('open', nowExp);
       document.getElementById('daytbl-' + dayId).hidden = !nowExp;