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/models/import.rb

@@ -0,0 +1,161 @@
+class Import < ApplicationRecord
+  belongs_to :account
+
+  validate :raw_csv_must_be_parsable
+
+  before_save :initialize_csv, if: :should_initialize_csv?
+
+  enum :status, { pending: "pending", complete: "complete", importing: "importing", failed: "failed" }, validate: true
+
+  store_accessor :column_mappings, :define_column_mapping_keys
+
+  scope :ordered, -> { order(created_at: :desc) }
+
+  def publish_later
+    ImportJob.perform_later(self)
+  end
+
+  def loaded?
+    raw_csv_str.present?
+  end
+
+  def configured?
+    csv.present?
+  end
+
+  def cleaned?
+    loaded? && configured? && csv.valid?
+  end
+
+  def csv
+    get_normalized_csv_with_validation
+  end
+
+  def available_headers
+    get_raw_csv.table.headers
+  end
+
+  def get_selected_header_for_field(field)
+    column_mappings&.dig(field) || field.key
+  end
+
+  def update_csv!(row_idx:, col_idx:, value:)
+    updated_csv = csv.update_cell(row_idx.to_i, col_idx.to_i, value)
+    update! normalized_csv_str: updated_csv.to_s
+  end
+
+  # Type-specific methods (potential STI inheritance in future when more import types added)
+  def publish
+    update!(status: "importing")
+
+    transaction do
+      generate_transactions.each do |txn|
+        txn.save!
+      end
+    end
+
+    update!(status: "complete")
+  rescue => e
+    update!(status: "failed")
+    Rails.logger.error("Import with id #{id} failed: #{e}")
+  end
+
+  def dry_run
+    generate_transactions
+  end
+
+  def expected_fields
+    @expected_fields ||= create_expected_fields
+  end
+
+  private
+
+    def get_normalized_csv_with_validation
+      return nil if normalized_csv_str.nil?
+
+      csv = Import::Csv.new(normalized_csv_str)
+
+      expected_fields.each do |field|
+        csv.define_validator(field.key, field.validator) if field.validator
+      end
+
+      csv
+    end
+
+    def get_raw_csv
+      return nil if raw_csv_str.nil?
+      Import::Csv.new(raw_csv_str)
+    end
+
+    def should_initialize_csv?
+      raw_csv_str_changed? || column_mappings_changed?
+    end
+
+    def initialize_csv
+      generated_csv = generate_normalized_csv(raw_csv_str)
+      self.normalized_csv_str = generated_csv.table.to_s
+    end
+
+    # Uses the user-provided raw CSV + mappings to generate a normalized CSV for the import
+    def generate_normalized_csv(csv_str)
+      Import::Csv.create_with_field_mappings(csv_str, expected_fields, column_mappings)
+    end
+
+    def update_csv(row_idx, col_idx, value)
+      updated_csv = csv.update_cell(row_idx.to_i, col_idx.to_i, value)
+      update! normalized_csv_str: updated_csv.to_s
+    end
+
+    def generate_transactions
+      transactions = []
+
+      csv.table.each do |row|
+        category = account.family.transaction_categories.find_or_initialize_by(name: row["category"])
+        txn = account.transactions.build \
+          name: row["name"] || "Imported transaction",
+          date: Date.iso8601(row["date"]),
+          category: category,
+          amount: BigDecimal(row["amount"]) * -1 # User inputs amounts with opposite signage of our internal representation
+
+        transactions << txn
+      end
+
+      transactions
+    end
+
+    def create_expected_fields
+      date_field = Import::Field.new \
+        key: "date",
+        label: "Date",
+        validator: ->(value) { Import::Field.iso_date_validator(value) }
+
+      name_field = Import::Field.new \
+        key: "name",
+        label: "Name"
+
+      category_field = Import::Field.new \
+        key: "category",
+        label: "Category"
+
+      amount_field = Import::Field.new \
+        key: "amount",
+        label: "Amount",
+        validator: ->(value) { Import::Field.bigdecimal_validator(value) }
+
+      [ date_field, name_field, category_field, amount_field ]
+    end
+
+    def define_column_mapping_keys
+      expected_fields.each do |field|
+        field.key.to_sym
+      end
+    end
+
+    def raw_csv_must_be_parsable
+      begin
+        CSV.parse(raw_csv_str || "")
+      rescue CSV::MalformedCSVError
+        errors.add(:raw_csv_str, "is not a valid CSV format")
+      end
+    end
+end