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/support/import_test_helper.rb

@@ -0,0 +1,24 @@
+module ImportTestHelper
+  def valid_csv_str
+    <<-ROWS
+      date,name,category,amount
+      2024-01-01,Starbucks drink,Food,20
+      2024-01-02,Amazon stuff,Shopping,200
+    ROWS
+  end
+
+  def valid_csv_with_invalid_values
+    <<-ROWS
+      date,name,category,amount
+      invalid_date,Starbucks drink,Food,invalid_amount
+    ROWS
+  end
+
+  def malformed_csv_str
+    <<-ROWS
+      name,age
+      "John Doe,23
+      "Jane Doe",25
+    ROWS
+  end
+end