Fix price chart destroyed by LiveView DOM patches on filter change

LiveView's morphdom was replacing the canvas element inside the chart
container whenever filters were toggled, destroying the Chart.js
rendering context. Add phx-update="ignore" to the chart container so
LiveView preserves JS-managed children after initial render.

lib/ammoprices_web/live/caliber_live/show.ex

@@ -282,6 +282,7 @@ defmodule AmmopricesWeb.CaliberLive.Show do
           <div
             id="price-chart-container"
             phx-hook=".PriceChart"
+            phx-update="ignore"
             data-chart-data={Jason.encode!(@chart_data)}
             class="h-64 sm:h-80"
           >

test/ammoprices_web/live/caliber_live/show_test.exs

@@ -182,6 +182,22 @@ defmodule AmmopricesWeb.CaliberLive.ShowTest do
       assert has_element?(view, "#filter-casing-steel")
     end
 
+    test "chart container uses phx-update=ignore to survive DOM patches", %{
+      conn: conn,
+      caliber: caliber,
+      retailer: retailer
+    } do
+      now = DateTime.truncate(DateTime.utc_now(), :second)
+      product = product_fixture(%{caliber: caliber, retailer: retailer, grain_weight: 115, in_stock: true})
+      snapshot_fixture(%{product: product, price_per_round_cents: 28, in_stock: true, recorded_at: now})
+
+      {:ok, view, _html} = live(conn, "/calibers/#{caliber.slug}")
+
+      # Chart container must have phx-update="ignore" so LiveView
+      # does not replace the canvas element managed by Chart.js
+      assert has_element?(view, "#price-chart-container[phx-update=ignore]")
+    end
+
     test "receives real-time price updates via PubSub", %{conn: conn, caliber: caliber, retailer: retailer} do
       now = DateTime.truncate(DateTime.utc_now(), :second)
       product = product_fixture(%{caliber: caliber, retailer: retailer, in_stock: true})