Add bulk Log Charged action to dashboard toolbar

Select multiple batteries, pick a date, and optionally increment charge
cycles for all of them in one shot. Reuses the existing bulk-action form
and POST /battery/bulk-action route; no new routes or migrations needed.

templates/dashboard.html

@@ -124,6 +124,16 @@
         <button class="btn btn-sm btn-primary" name="action" value="install_device" type="submit"
                 onclick="return confirmInstallDevice()">Install in device</button>
       </span>
+      <span style="display:flex;gap:0.35rem;align-items:center;flex-wrap:wrap;">
+        <input type="date" name="charged_date" id="bulk-charged-date"
+               style="padding:0.25rem 0.5rem;font-size:0.85rem;border:1px solid #cbd5e1;border-radius:4px;">
+        <label style="font-size:0.85rem;display:flex;align-items:center;gap:0.25rem;cursor:pointer;">
+          <input type="checkbox" name="increment_cycles" id="bulk-increment-cycles" value="1" checked>
+          +cycle
+        </label>
+        <button class="btn btn-sm btn-primary" name="action" value="log_charged" type="submit"
+                onclick="return validateBulkCharge()">Log Charged</button>
+      </span>
     </div>
 
     <div class="table-wrap">
@@ -349,6 +359,12 @@ document.addEventListener('click', function() {
   document.getElementById('col-picker-panel').style.display = 'none';
 });
 
+function validateBulkCharge() {
+  var d = document.getElementById('bulk-charged-date');
+  if (!d.value) { d.focus(); return false; }
+  return true;
+}
+
 function bulkStorageChanged(sel) {
   var text = document.getElementById('bulk-storage-text');
   var hidden = document.getElementById('bulk-field-value-storage');