Bulk upload / CSV import for Fixed and Variable Costs

Currently, adding fixed and variable costs requires entering each line item manually one at a time. For businesses with many recurring expenses (subscriptions, contractors, tools, etc.), this is extremely time-consuming to set up especially when doing initial historical configuration.

Feature request: allow bulk upload of fixed and variable costs via CSV, similar to how shipping costs can already be imported. A simple CSV format with columns like name, type, amount, interval, start_date, end_date would cover the majority of use cases and make onboarding significantly faster.

This would also make it much easier to audit and update costs over time by exporting, editing in a spreadsheet, and re-importing.