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/imports/_empty.html.erb

@@ -0,0 +1,9 @@
+<div class="flex justify-center items-center py-20">
+  <div class="text-center flex flex-col items-center max-w-[300px]">
+    <p class="text-gray-900 mb-1 font-medium text-sm"><%= t(".message") %></p>
+    <%= link_to new_import_path(enable_type_selector: true), class: "w-fit flex text-white text-sm font-medium items-center gap-1 bg-gray-900 rounded-lg p-2 pr-3", data: { turbo_frame: "modal" } do %>
+      <%= lucide_icon("plus", class: "w-5 h-5") %>
+      <span><%= t(".new") %></span>
+    <% end %>
+  </div>
+</div>