feat(splits): add exclusion support for splits and improve rendering (#1661)

* feat(splits): add excluded attribute support for split children and improve rendering of split transactions

* address coderabbitai suggestions to improve code quality

* Fix split excluded coercion, DRY helpers, and clean up view partials

Fix boolean coercion bug where string "false" from form params was
truthy in Ruby, causing all split children to be marked excluded.
Use ActiveModel::Type::Boolean for explicit casting in Entry#split!.

Additional changes addressing code review feedback:

- Extract duplicated in_split_group logic from TransactionsController
  and TransactionCategoriesController into TransactionsHelper
- Remove redundant local_assigns.fetch calls in partials that already
  declare defaults via the Rails 7.1 locals: magic comment
- Simplify ternary in _transaction.html.erb to pass grouped directly
- Guard hidden_field_tag :grouped to only emit when value is "true"
- Add model tests for excluded on split children (boolean and string)
- Add controller test for excluded param through full HTTP stack
- Add test confirming excluded children are dropped from balance queries

* fix(splits): simplify excluded attribute boolean check

* refactor(splits): extract truthy values constant for excluded check

Extract the array of truthy values used for excluded attribute check
into a private constant to improve code maintainability and avoid
duplication of the magic array.

* refactor: simplify split grouping link generation and add test coverage for excluded split parameters

app/models/entry.rb

@@ -1,6 +1,9 @@
 class Entry < ApplicationRecord
   include Monetizable, Enrichable
 
+  TRUTHY_VALUES = [ true, "true", "1", 1 ].freeze
+  private_constant :TRUTHY_VALUES
+
   attr_accessor :unsplitting
 
   monetize :amount
@@ -361,7 +364,7 @@ class Entry < ApplicationRecord
 
   # Splits this entry into child entries. Marks parent as excluded.
   #
-  # @param splits [Array<Hash>] array of { name:, amount:, category_id: } hashes
+  # @param splits [Array<Hash>] array of { name:, amount:, category_id:, excluded: } hashes
   # @return [Array<Entry>] the created child entries
   def split!(splits)
     total = splits.sum { |s| s[:amount].to_d }
@@ -383,6 +386,7 @@ class Entry < ApplicationRecord
           name: split_attrs[:name],
           amount: split_attrs[:amount],
           currency: currency,
+          excluded: TRUTHY_VALUES.include?(split_attrs[:excluded]),
           entryable: child_transaction
         )
       end