feat(retirement): PR4g happy-path system test

One system test: a preview-enabled user visits /retirement (sees the
"add your birth year" prompt), fills the birth-year + retire-age levers,
saves, and the KPI cards + D3 glide chart render. Asserts the rendered
SVG + KPI container (labels are CSS-uppercased, so we match structure,
not the case-folded text).

test/system/retirement_test.rb

@@ -0,0 +1,27 @@
+require "application_system_test_case"
+
+class RetirementTest < ApplicationSystemTestCase
+  setup do
+    @user = users(:family_admin)
+    @user.update!(preferences: (@user.preferences || {}).merge("preview_features_enabled" => true))
+    @user.family.update!(retirement_disabled: false)
+    sign_in @user
+  end
+
+  test "set up a plan and see the projection render" do
+    visit retirement_path
+
+    assert_selector "h1", text: I18n.t("retirement.show.title")
+    assert_text I18n.t("retirement.kpis.set_birth_year_heading")
+
+    fill_in "retirement[birth_year]", with: (Date.current.year - 40).to_s
+    fill_in "retirement[retire_age]", with: "55"
+    click_button I18n.t("retirement.what_if.save")
+
+    # After persisting, the forecast is projectable: the KPI cards + the D3
+    # glide chart render. (The KPI label is CSS-uppercased, so match the
+    # rendered SVG + the KPI container rather than the case-folded label.)
+    assert_selector "#retirement_kpis", wait: 5
+    assert_selector "[data-controller='retirement-glide-chart'] svg", wait: 5
+  end
+end