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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Computer Software and Website Development (Q1) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Dec. 31, 2022
Software development costs        
Property, Plant and Equipment [Line Items]        
Property, plant and equipment, useful life 3 years   3 years  
Internal software development costs incurred during the period $ 0   $ 323 $ 114
Website development costs        
Property, Plant and Equipment [Line Items]        
Property, plant and equipment, useful life 3 years   3 years  
Internal software development costs incurred during the period $ 56   $ 241 114
Capitalized computer software, amortization expense $ 26 $ 15 $ 56 $ 14