Add bulk install-in-device from dashboard and unretire action

- Dashboard bulk toolbar: select batteries, pick a device, click 'Install
  in device'; confirms before moving already-installed batteries; enforces
  slot capacity and warns on brand mix
- Battery detail: 'Unretire Battery' button replaces 'Retire Battery' when
  battery is retired, restoring it to available status
- Tests: 3 new bulk-install-device tests (capacity block, move, success);
  42 total passing

templates/battery_detail.html

@@ -185,6 +185,10 @@
       <form method="post" action="{{ url_for('battery_retire', battery_id=battery.id) }}">
         <button class="btn btn-secondary" type="submit">Retire Battery</button>
       </form>
+    {% else %}
+      <form method="post" action="{{ url_for('battery_unretire', battery_id=battery.id) }}">
+        <button class="btn btn-secondary" type="submit">Unretire Battery</button>
+      </form>
     {% endif %}
 
     <a class="btn btn-danger" href="{{ url_for('battery_delete', battery_id=battery.id) }}">Delete Battery</a>

templates/dashboard.html

@@ -113,6 +113,17 @@
         </span>
         <button class="btn btn-sm btn-primary" name="action" value="set_field" type="submit">Apply</button>
       </span>
+      <span style="display:flex;gap:0.35rem;align-items:center;">
+        <select id="bulk-device-select" name="device_id"
+                style="padding:0.25rem 0.5rem;font-size:0.85rem;border:1px solid #cbd5e1;border-radius:4px;">
+          <option value="">— select device —</option>
+          {% for d in devices %}
+          <option value="{{ d.id }}">{{ d.name }} ({{ d.installed_count() }}/{{ d.battery_slots }})</option>
+          {% endfor %}
+        </select>
+        <button class="btn btn-sm btn-primary" name="action" value="install_device" type="submit"
+                onclick="return confirmInstallDevice()">Install in device</button>
+      </span>
     </div>
 
     <div class="table-wrap">
@@ -260,6 +271,23 @@ function applyFilters() {
   updateToolbar();
 }
 
+function confirmInstallDevice() {
+  var deviceSel = document.getElementById('bulk-device-select');
+  if (!deviceSel.value) { deviceSel.focus(); return false; }
+  var movers = Array.prototype.filter.call(
+    document.querySelectorAll('.row-cb:checked'),
+    function(cb) { return cb.closest('tr').dataset.status === 'installed'; }
+  );
+  if (movers.length > 0) {
+    var n = movers.length;
+    return confirm(
+      n + ' selected batter' + (n === 1 ? 'y is' : 'ies are') +
+      ' already installed elsewhere. Unassign and move to the selected device?'
+    );
+  }
+  return true;
+}
+
 function quickAssign(action, batteryId) {
   var sel = document.getElementById('qas-' + batteryId);
   if (!sel.value) { sel.focus(); return; }