Refactor transaction merging logic to prefer accurate posted dates and improve deduplication (#449)

- Updated `date_utils.rb` to return `nil` for timestamps equal to 0.
- Enhanced `SimplefinItem::Importer` to merge transactions by prioritizing non-pending records with valid posted timestamps.
- Introduced a comparator and key-builder for resolving transaction conflicts systematically.

Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

lib/simplefin/date_utils.rb

@@ -15,6 +15,7 @@ module Simplefin
       when Time, DateTime
         val.to_date
       when Integer, Float
+        return nil if val.to_i == 0
         Time.at(val).utc.to_date
       when String
         Date.parse(val)