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/battery_detail.html

@@ -114,7 +114,7 @@
           <td data-label="">
             <form class="inline" method="post"
                   action="{{ url_for('battery_capacity_test_delete', battery_id=battery.id, test_id=t.id) }}"
-                  onsubmit="return confirm('Delete this test record?')">
+                  data-confirm="Delete this test record?" data-confirm-ok="Delete">
               <button class="btn btn-sm btn-danger" type="submit">Delete</button>
             </form>
           </td>
@@ -172,7 +172,7 @@
           <td data-label="">
             <form class="inline" method="post"
                   action="{{ url_for('battery_charge_log_delete', battery_id=battery.id, log_id=log.id) }}"
-                  onsubmit="return confirm('Delete this charge log entry?')">
+                  data-confirm="Delete this charge log entry?" data-confirm-ok="Delete">
               <button class="btn btn-sm btn-danger" type="submit">Delete</button>
             </form>
           </td>