Add inline assign from dashboard, specific battery picker on device, dynamic install rows

- Dashboard: replace Assign link with device dropdown + arrow button for
  quick inline assignment without leaving the page
- Device detail: replace hardcoded 4-row install form with 1 row + '+ Add
  brand' button that clones rows dynamically
- Device detail: add 'Install Specific Battery' card with dropdown of all
  available batteries (label, brand, size, notes) via new /device/<id>/install-one route
- Tests: 4 new acceptance tests covering dashboard quick-assign and
  install-one, including capacity enforcement on both paths (39 total)

templates/dashboard.html

@@ -167,7 +167,18 @@
               <a class="btn btn-sm btn-secondary" href="{{ url_for('battery_detail', battery_id=b.id) }}">View</a>
 
               {% if b.is_available() %}
-                <a class="btn btn-sm btn-primary" href="{{ url_for('battery_assign', battery_id=b.id) }}">Assign</a>
+                <select id="qas-{{ b.id }}"
+                        style="padding:0.2rem 0.3rem;font-size:0.8rem;border:1px solid #cbd5e1;border-radius:4px;max-width:110px;vertical-align:middle;">
+                  <option value="">— assign —</option>
+                  {% for d in devices %}
+                  <option value="{{ d.id }}"
+                    {% if d.installed_count() >= d.battery_slots %}disabled{% endif %}>
+                    {{ d.name }} ({{ d.installed_count() }}/{{ d.battery_slots }})
+                  </option>
+                  {% endfor %}
+                </select>
+                <button type="button" class="btn btn-sm btn-primary"
+                        onclick="quickAssign('{{ url_for('battery_assign', battery_id=b.id) }}', {{ b.id }})">→</button>
               {% endif %}
 
               {% if b.is_installed() %}
@@ -249,6 +260,18 @@ function applyFilters() {
   updateToolbar();
 }
 
+function quickAssign(action, batteryId) {
+  var sel = document.getElementById('qas-' + batteryId);
+  if (!sel.value) { sel.focus(); return; }
+  var f = document.createElement('form');
+  f.method = 'post'; f.action = action;
+  var inp = document.createElement('input');
+  inp.type = 'hidden'; inp.name = 'device_id'; inp.value = sel.value;
+  f.appendChild(inp);
+  document.body.appendChild(f);
+  f.submit();
+}
+
 function resetFilters() {
   ['filter-status','filter-brand','filter-size','filter-storage'].forEach(function(id) {
     document.getElementById(id).value = '';