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/fixtures/imports.yml

@@ -0,0 +1,32 @@
+empty_import:
+  account: checking
+  created_at: <%= 1.minute.ago %>
+
+loaded_import:
+  account: checking
+  raw_csv_str: |
+    date,name,category,amount
+    2024-01-01,Starbucks drink,Food,20
+    2024-01-02,Amazon stuff,Shopping,200
+  normalized_csv_str: |
+    date,name,category,amount
+    2024-01-01,Starbucks drink,Food,20
+    2024-01-02,Amazon stuff,Shopping,200
+  created_at: <%= 2.days.ago %>
+
+completed_import:
+  account: checking
+  column_mappings:
+    date: date
+    name: name
+    category: category
+    amount: amount
+  raw_csv_str: |
+    date,name,category,amount
+    2024-01-01,Starbucks drink,Food,20
+  normalized_csv_str: |
+    date,name,category,amount
+    2024-01-01,Starbucks drink,Food,20
+  created_at: <%= 2.days.ago %>
+
+