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

app.py

@@ -95,8 +95,14 @@ def create_app(config_object="config"):
             .filter(Battery.storage_location.isnot(None))
             .distinct().order_by(Battery.storage_location).all()
         ]
+        brand_counts = {
+            r[0]: r[1]
+            for r in db.query(Battery.brand, func.count(Battery.id))
+                       .group_by(Battery.brand).all()
+        }
         return render_template("battery_add.html", form_count=1, brands=brands,
-                               storage_locations=storage_locations)
+                               storage_locations=storage_locations,
+                               brand_counts=brand_counts)
 
     # ------------------------------------------------------------------ #
     # Battery — detail