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/index.html.erb

@@ -0,0 +1,30 @@
+<% content_for :sidebar do %>
+  <%= render "settings/nav" %>
+<% end %>
+
+<div class="space-y-4">
+  <div class="flex items-center justify-between">
+    <h1 class="text-xl font-medium text-gray-900"><%= t(".title") %></h1>
+    <%= link_to new_import_path(enable_type_selector: true), class: "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 class="bg-white shadow-xs border border-alpha-black-25 rounded-xl p-4">
+    <% if @imports.empty? %>
+      <%= render partial: "imports/empty" %>
+    <% else %>
+      <div class="rounded-xl bg-gray-25 p-1">
+        <h2 class="uppercase px-4 py-2 text-gray-500 text-xs"><%= t(".imports") %> · <%= @imports.size %></h2>
+
+        <div class="border border-alpha-gray-100 rounded-lg bg-white shadow-xs">
+          <%= render @imports.ordered %>
+        </div>
+      </div>
+    <% end %>
+  </div>
+  <div class="flex justify-between gap-4">
+    <%= previous_setting("Rules", transaction_rules_path) %>
+    <%= next_setting("What's new", changelog_path) %>
+  </div>
+</div>