feat: Implement Split Transactions

[hendripermana]

Summary

Implements split transactions feature, allowing users to break a single transaction into multiple categorized sub-entries while maintaining the original total amount.

Changes

Database

• Migration: Added parent_entry_id column to entries table with foreign key and index

Models

• Entry: Added split!, unsplit!, update_parent_from_splits! methods, split_children/parent_entry associations, validations for split integrity
• Transaction: Added splittable? helper
• User: Added split_transaction/unsplit_transaction permission methods

Controllers

• SplitsController: Full CRUD for split entries (new, create, edit, update, destroy)
• TransactionsController: Added split/unsplit actions

Views

• Split creation and editing forms with dynamic row management
• Split group display partial for transaction detail view
• Parent row indicator in transaction list
• Updated transaction show page with split details

JavaScript

• split_transaction_controller.js: Stimulus controller for dynamic split form management (add/remove rows, real-time validation, amount distribution)

Tests

• entry_split_test.rb: 15 model tests covering split creation, validation, amount integrity, and edge cases
• splits_controller_test.rb: 17 controller tests covering CRUD operations, authorization, and error handling

Security

• ✅ Brakeman scan: Clean
• ✅ RuboCop: Compliant
• ✅ All tests passing

How to Test

1. Run migration: bin/rails db:migrate
2. Navigate to any transaction detail page
3. Click "Split Transaction" to create sub-entries
4. Verify amounts must sum to the original total
5. Test unsplit to restore the original transaction