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

tests/test_acceptance.py

@@ -41,6 +41,24 @@ def test_dashboard_loads(seeded_client):
     assert b"BrandX 002" in resp.data
 
 
+# ------------------------------------------------------------------ #
+# Battery add — label preview data
+# ------------------------------------------------------------------ #
+
+def test_battery_add_get_renders(client):
+    resp = client.get("/battery/add")
+    assert resp.status_code == 200
+    assert b"brandCounts" in resp.data
+
+
+def test_battery_add_brand_counts_reflects_existing(seeded_client):
+    # seeded_client has BrandX x2, BrandY x1
+    resp = seeded_client.get("/battery/add")
+    assert resp.status_code == 200
+    assert b'"BrandX": 2' in resp.data or b'"BrandX":2' in resp.data
+    assert b'"BrandY": 1' in resp.data or b'"BrandY":1' in resp.data
+
+
 # ------------------------------------------------------------------ #
 # Battery — bulk add
 # ------------------------------------------------------------------ #