audit pass 8: keyboard accessibility and table semantics

CSS:
- Fix light theme input/select/textarea :focus -> :focus-visible

HTML:
- Worker metrics table: convert label <td> to <th scope="row"> for screen readers
- Add aria-label to worker metrics table
- Sticky table: add scope="col" to all column headers
- Keyboard shortcuts modal table: add scope="col" to headers
- Kanban cards: remove tabindex="0" from role="article" (non-interactive)
- Advanced filter: ensure all 3 label/select pairs have for/id associations

JS:
- Lightbox: fix keydown listener leak by storing bound reference for removeEventListener
- Lightbox: save/restore trigger focus on open/close
- Sortable table: add tabindex="0" and Enter/Space keydown handler on sortable <th>
- Split pane: add tabindex="0", role="separator", aria-label, and arrow/Home/End
  keyboard resize support on divider (5% steps)
- Form validation: handle <select multiple> required check via selectedOptions.length

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

base.js

@@ -558,7 +558,8 @@
     ths.forEach((th, colIdx) => {
       let dir = 'asc';
       th.setAttribute('aria-sort', 'none');
-      th.addEventListener('click', () => {
+      th.setAttribute('tabindex', '0');
+      const _sort = () => {
         ths.forEach(h => { h.removeAttribute('data-sort'); h.setAttribute('aria-sort', 'none'); });
         th.setAttribute('data-sort', dir);
         th.setAttribute('aria-sort', dir === 'asc' ? 'ascending' : 'descending');
@@ -573,7 +574,9 @@
         });
         rows.forEach(r => tbody.appendChild(r));
         dir = dir === 'asc' ? 'desc' : 'asc';
-      });
+      };
+      th.addEventListener('click', _sort);
+      th.addEventListener('keydown', e => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); _sort(); } });
     });
   }
 
@@ -993,6 +996,7 @@
   function _validateField(el) {
     const val = el.value || '', type = (el.type || '').toLowerCase();
     if ((type === 'checkbox' || type === 'radio') && el.required && !el.checked) return { valid: false, message: 'This field is required' };
+    if (el.tagName === 'SELECT' && el.multiple && el.required && el.selectedOptions.length === 0) return { valid: false, message: 'Please select at least one option' };
     if (el.required && !val.trim())                                             return { valid: false, message: 'This field is required' };
     if (el.minLength > 0 && val.length < el.minLength)                         return { valid: false, message: 'Minimum ' + el.minLength + ' characters' };
     if (el.maxLength > 0 && val.length > el.maxLength)                         return { valid: false, message: 'Maximum ' + el.maxLength + ' characters' };
@@ -2133,6 +2137,25 @@
       });
       divider.addEventListener('pointerup', () => { dragging = false; divider.classList.remove('is-dragging'); });
 
+      // Keyboard resize support
+      divider.setAttribute('tabindex', '0');
+      divider.setAttribute('role', 'separator');
+      divider.setAttribute('aria-label', 'Resize panes');
+      divider.addEventListener('keydown', e => {
+        const step = 0.05;
+        const total = vertical ? container.clientHeight : container.clientWidth;
+        const divSize = vertical ? divider.offsetHeight : divider.offsetWidth;
+        const available = total - divSize;
+        const currentSize = vertical ? panes[0].offsetHeight : panes[0].offsetWidth;
+        const currentRatio = currentSize / available;
+        if ((e.key === 'ArrowRight' && !vertical) || (e.key === 'ArrowDown' && vertical)) {
+          e.preventDefault(); _setRatio(Math.min(1, currentRatio + step));
+        } else if ((e.key === 'ArrowLeft' && !vertical) || (e.key === 'ArrowUp' && vertical)) {
+          e.preventDefault(); _setRatio(Math.max(0, currentRatio - step));
+        } else if (e.key === 'Home') { e.preventDefault(); _setRatio(0); }
+        else if (e.key === 'End')   { e.preventDefault(); _setRatio(1); }
+      });
+
       _setRatio(initial);
       return { setRatio: _setRatio };
     },
@@ -2477,7 +2500,7 @@
   const lightbox = {
     init(selector, opts = {}) {
       const { caption = 'alt', loop = true } = opts;
-      let _images = [], _current = 0, _overlay = null;
+      let _images = [], _current = 0, _overlay = null, _lbKeyBound = null, _lbTrigger = null;
 
       function _getCaption(img) {
         if (typeof caption === 'function') return caption(img);
@@ -2507,7 +2530,8 @@
         _overlay.querySelector('.lt-lightbox-prev').addEventListener('click', () => lightbox.prev());
         _overlay.querySelector('.lt-lightbox-next').addEventListener('click', () => lightbox.next());
         _overlay.addEventListener('click', e => { if (e.target === _overlay) lightbox.close(); });
-        document.addEventListener('keydown', _lbKey);
+        _lbKeyBound = _lbKey.bind(null);
+        document.addEventListener('keydown', _lbKeyBound);
       }
 
       function _lbKey(e) {
@@ -2519,6 +2543,9 @@
 
       function _show(idx) {
         if (!_overlay) _buildOverlay();
+        if (!_overlay.classList.contains('is-open')) {
+          _lbTrigger = (document.activeElement && document.activeElement !== document.body) ? document.activeElement : null;
+        }
         _current = idx;
         const img = _images[idx];
         const el  = _overlay.querySelector('.lt-lightbox-img');
@@ -2554,6 +2581,8 @@
           if (!_overlay) return;
           _overlay.classList.remove('is-open');
           _unlockScroll();
+          if (_lbTrigger && document.contains(_lbTrigger)) { _lbTrigger.focus(); }
+          _lbTrigger = null;
         },
         prev()   { _show(loop ? (_current - 1 + _images.length) % _images.length : Math.max(0, _current - 1)); },
         next()   { _show(loop ? (_current + 1) % _images.length : Math.min(_images.length - 1, _current + 1)); },