CSV Transaction Imports (#708)

Introduces a basic CSV import module for bulk-importing account transactions.

Changes include:

- User can load a CSV
- User can configure the column mappings for a CSV
- Imported CSV shows invalid cells
- User can clean up their data directly in the UI
- User can see a preview of the import rows and confirm import
- Layout refactor + Import nav stepper
- System test stability improvements

test/system/imports_test.rb

@@ -0,0 +1,113 @@
+require "application_system_test_case"
+
+class ImportsTest < ApplicationSystemTestCase
+  include ImportTestHelper
+
+  setup do
+    sign_in @user = users(:family_admin)
+
+    @imports = @user.family.imports.ordered.to_a
+  end
+
+  test "can trigger new import from settings" do
+    trigger_import_from_settings
+    verify_import_modal
+  end
+
+  test "can resume existing import from settings" do
+    visit imports_url
+
+    within "#" + dom_id(@imports.first) do
+      click_button
+      click_link "Edit"
+    end
+
+    assert_current_path edit_import_path(@imports.first)
+  end
+
+  test "can resume latest import" do
+    trigger_import_from_transactions
+    verify_import_modal
+
+    click_link "Resume latest import"
+
+    assert_current_path edit_import_path(@imports.first)
+  end
+
+  test "can perform basic CSV import" do
+    trigger_import_from_settings
+    verify_import_modal
+
+    within "#modal" do
+      click_link "New import from CSV"
+    end
+
+    # 1) Create import step
+    assert_selector "h1", text: "New import"
+
+    within "form" do
+      select "Checking Account", from: "import_account_id"
+    end
+
+    click_button "Next"
+
+    # 2) Load Step
+    assert_selector "h1", text: "Load import"
+
+    within "form" do
+      fill_in "import_raw_csv_str", with: <<-ROWS
+        date,Custom Name Column,category,amount
+        invalid_date,Starbucks drink,Food,-20.50
+        2024-01-01,Amazon purchase,Shopping,-89.50
+      ROWS
+    end
+
+    click_button "Next"
+
+    # 3) Configure step
+    assert_selector "h1", text: "Configure import"
+
+    within "form" do
+      select "Custom Name Column", from: "import_column_mappings_name"
+    end
+
+    click_button "Next"
+
+    # 4) Clean step
+    assert_selector "h1", text: "Clean import"
+
+    # We have an invalid value, so user cannot click next yet
+    assert_no_text "Next"
+
+    # Replace invalid date with valid date
+    fill_in "cell-0-0", with: "2024-01-02"
+
+    # Trigger blur event so value saves
+    find("body").click
+
+    click_link "Next"
+
+    # 5) Confirm step
+    assert_selector "h1", text: "Confirm import"
+    click_button "Import 2 transactions"
+    assert_selector "h1", text: "Imports"
+  end
+
+  private
+
+    def trigger_import_from_settings
+      visit imports_url
+      click_link "New import"
+    end
+
+    def trigger_import_from_transactions
+      visit transactions_url
+      click_link "Import"
+    end
+
+    def verify_import_modal
+      within "#modal" do
+        assert_text "Import transactions"
+      end
+    end
+end