Add Select all/Deselect all button to filter bar for mobile

templates/dashboard.html

@@ -70,6 +70,7 @@
     </select>
     <input type="text" id="filter-text" oninput="applyFilters()" placeholder="Search…"
            style="padding:0.25rem 0.5rem;font-size:0.85rem;border:1px solid #cbd5e1;border-radius:4px;width:140px;">
+    <button type="button" id="select-all-btn" onclick="mobileSelectAll()" class="btn btn-sm btn-secondary" style="display:none;">Select all</button>
     <button type="button" onclick="resetFilters()" class="btn btn-sm btn-secondary" id="filter-reset" style="display:none;">✕ Reset</button>
     <span id="filter-count" style="font-size:0.8rem;color:#64748b;"></span>
   </div>
@@ -224,6 +225,7 @@
 var selectAll = document.getElementById('select-all');
 var toolbar = document.getElementById('bulk-toolbar');
 var countEl = document.getElementById('selected-count');
+var selectAllBtn = document.getElementById('select-all-btn');
 
 function visibleCbs() {
   return Array.prototype.filter.call(
@@ -241,6 +243,18 @@ function updateToolbar() {
   var visChecked = vis.filter(function(cb) { return cb.checked; });
   selectAll.indeterminate = visChecked.length > 0 && visChecked.length < vis.length;
   selectAll.checked = vis.length > 0 && visChecked.length === vis.length;
+  if (selectAllBtn) {
+    selectAllBtn.style.display = vis.length > 0 ? '' : 'none';
+    selectAllBtn.textContent = (vis.length > 0 && visChecked.length === vis.length)
+      ? 'Deselect all' : 'Select all';
+  }
+}
+
+function mobileSelectAll() {
+  var vis = visibleCbs();
+  var allChecked = vis.length > 0 && vis.every(function(cb) { return cb.checked; });
+  vis.forEach(function(cb) { cb.checked = !allChecked; });
+  updateToolbar();
 }
 
 document.querySelectorAll('.row-cb').forEach(function(cb) {
@@ -340,6 +354,7 @@ function updateBulkField(sel) {
 }
 // initialise disabled state on page load
 document.getElementById('bulk-field-value-brand').disabled = true;
+updateToolbar();
 
 // Column picker
 var COL_KEY = 'battery_cols';