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

app/views/settings/_nav.html.erb

@@ -55,6 +55,9 @@
     <li>
       <%= sidebar_link_to t(".rules_label"), transaction_rules_path, icon: "list-checks" %>
     </li>
+    <li>
+      <%= sidebar_link_to t(".imports_label"), imports_path, icon: "download" %>
+    </li>
   </ul>
 </section>
 <section class="space-y-2">