Replace browser confirm() dialogs with custom modal; add live label preview on battery add form

- base.html: add CSS/HTML/JS for styled in-app confirmation modal (dark-mode compatible via CSS vars)
- device_list, battery_detail: convert onsubmit confirm() to declarative data-confirm attributes
- dashboard: convert bulk Delete/Install buttons to use modal helpers (submitWithAction pattern)
- app.py: pass brand_counts dict to battery_add template
- battery_add.html: show live "Will create: Brand 001 → Brand 003" preview as brand/quantity change
- tests: add two tests covering brand_counts server-side rendering

templates/device_list.html

@@ -59,12 +59,14 @@
             <a class="btn btn-sm btn-secondary" href="{{ url_for('device_detail', device_id=d.id) }}">View</a>
             {% if d.installed_count() > 0 %}
             <form class="inline" method="post" action="{{ url_for('device_unassign_all', device_id=d.id) }}"
-                  onsubmit="return confirm('Unassign all batteries from {{ d.name }}?')">
+                  data-confirm="Unassign all batteries from {{ d.name }}?"
+                  data-confirm-ok="Unassign" data-confirm-class="btn-warning">
               <button class="btn btn-sm btn-warning" type="submit">Unassign All</button>
             </form>
             {% endif %}
             <form class="inline" method="post" action="{{ url_for('device_delete', device_id=d.id) }}"
-                  onsubmit="return confirm('Delete {{ d.name }}? All installed batteries will be unassigned.');">
+                  data-confirm="Delete {{ d.name }}? All installed batteries will be unassigned."
+                  data-confirm-ok="Delete" data-confirm-class="btn-danger">
               <button class="btn btn-sm btn-danger" type="submit">Delete</button>
             </form>
           </td>