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/models/import/csv_test.rb

@@ -0,0 +1,87 @@
+require "test_helper"
+
+class Import::CsvTest < ActiveSupport::TestCase
+  include ImportTestHelper
+
+  setup do
+    @csv = Import::Csv.new(valid_csv_str)
+  end
+
+  test "cannot define validator for non-existent header" do
+    assert_raises do
+      @csv.define_validator "invalid", method(:validate_iso_date)
+    end
+  end
+
+  test "csv with no validators is valid" do
+    assert @csv.cell_valid?(0, 0)
+    assert @csv.valid?
+  end
+
+  test "valid csv values" do
+    @csv.define_validator "date", method(:validate_iso_date)
+
+    assert_equal "2024-01-01", @csv.table[0][0]
+    assert @csv.cell_valid?(0, 0)
+    assert @csv.valid?
+  end
+
+  test "invalid csv values" do
+    invalid_csv = Import::Csv.new valid_csv_with_invalid_values
+
+    invalid_csv.define_validator "date", method(:validate_iso_date)
+
+    assert_equal "invalid_date", invalid_csv.table[0][0]
+    assert_not invalid_csv.cell_valid?(0, 0)
+    assert_not invalid_csv.valid?
+  end
+
+  test "updating a cell returns a copy of the original csv" do
+    original_date = "2024-01-01"
+    new_date = "2024-01-01"
+
+    assert_equal original_date, @csv.table[0][0]
+    updated = @csv.update_cell(0, 0, new_date)
+
+    assert_equal original_date, @csv.table[0][0]
+    assert_equal new_date, updated[0][0]
+  end
+
+  test "can create CSV with expected columns and field mappings with validators" do
+    date_field = Import::Field.new \
+      key: "date",
+      label: "Date",
+      validator: method(:validate_iso_date)
+
+    name_field = Import::Field.new \
+      key: "name",
+      label: "Name"
+
+    fields = [ date_field, name_field ]
+
+    raw_csv_str = <<-ROWS
+      date,Custom Field Header,extra_field
+      invalid_date_value,Starbucks drink,Food
+      2024-01-02,Amazon stuff,Shopping
+    ROWS
+
+    mappings = {
+      "name" => "Custom Field Header"
+    }
+
+    csv = Import::Csv.create_with_field_mappings(raw_csv_str, fields, mappings)
+
+    assert_equal %w[ date name ], csv.table.headers
+    assert_equal 2, csv.table.size
+    assert_equal "Amazon stuff", csv.table[1][1]
+  end
+
+  private
+
+    def validate_iso_date(value)
+      Date.iso8601(value)
+      true
+    rescue
+      false
+    end
+end