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/load.html.erb

@@ -0,0 +1,39 @@
+<%= content_for :return_to_path, return_to_path(params, imports_path) %>
+
+<div class="mx-auto max-w-[450px] w-full py-24 space-y-4">
+  <h1 class="sr-only"><%= t(".load_title") %></h1>
+
+  <div class="text-center space-y-2">
+    <h2 class="text-3xl text-gray-900 font-medium"><%= t(".subtitle") %></h2>
+    <p class="text-gray-500 text-sm"><%= t(".description") %></p>
+  </div>
+
+  <%= form_with model: @import, url: load_import_path(@import) do |form| %>
+    <div>
+      <%= form.text_area :raw_csv_str,
+                         rows: 10,
+                         required: true,
+                         placeholder: "Paste your CSV file contents here",
+                         class: "rounded-md w-full border text-sm border-alpha-black-100 bg-white placeholder:text-gray-400 p-4" %>
+    </div>
+
+    <%= form.submit t(".next"), class: "px-4 py-2 block w-full rounded-lg bg-gray-900 text-white text-sm font-medium", data: { turbo_confirm: (@import.raw_csv_str? ? { title: t(".confirm_title"), body: t(".confirm_body"), accept: t(".confirm_accept") } : nil) } %>
+  <% end %>
+
+  <div class="bg-alpha-black-25 rounded-xl p-1">
+    <div class="text-gray-500 p-2 mb-2">
+      <div class="flex gap-2 mb-2">
+        <%= lucide_icon("info", class: "w-5 h-5 shrink-0") %>
+        <p class="text-sm"><%= t(".instructions") %></p>
+      </div>
+
+      <ul class="list-disc text-sm pl-10">
+        <li><%= t(".requirement1") %></li>
+        <li><%= t(".requirement2") %></li>
+      </ul>
+    </div>
+
+    <%= render partial: "imports/sample_table" %>
+
+  </div>
+</div>