Add StatisticsLive: overview, bars by year, pie chart

- Summary cards: active/inactive members, open amount
- Joins and exits by year (horizontal bars)
- Contributions by year: table with stacked bar above amounts
- Column order: Paid, Unpaid, Suspended, Total; color dots for legend
- All years combined pie chart
- LiveView tests

test/mv_web/live/statistics_live_test.exs

@@ -0,0 +1,32 @@
+defmodule MvWeb.StatisticsLiveTest do
+  @moduledoc """
+  Tests for the Statistics LiveView at /statistics.
+  """
+  use MvWeb.ConnCase, async: true
+
+  import Phoenix.LiveViewTest
+
+  describe "statistics page" do
+    test "renders statistics page with title and key labels for authenticated user with access",
+         %{
+           conn: conn
+         } do
+      {:ok, _view, html} = live(conn, ~p"/statistics")
+
+      assert html =~ "Statistics"
+      assert html =~ "Active members"
+      assert html =~ "Open amount"
+      assert html =~ "Contributions by year"
+      assert html =~ "Joins and exits by year"
+    end
+
+    test "page shows overview of all relevant years without year selector", %{conn: conn} do
+      {:ok, _view, html} = live(conn, ~p"/statistics")
+
+      # No year dropdown: single select for year should not be present as main control
+      assert html =~ "Overview" or html =~ "overview"
+      # table header or legend
+      assert html =~ "Year"
+    end
+  end
+end