This commit updates all instances of 'base' line item type to 'flat'. It modifies the BillingIntervalSchema, the validation rules for one-time plans, and the function to get the primary line item for a plan. Furthermore, it adjusts the display and filtering of line items in the pricing table component and the plan picker component. The SQL migration script and the sample billing configuration are also updated to reflect this change.
Updated the billing schema to include a more descriptive line item and an optional tiers element. Also, billing configuration was refactored and displayed more prominently in the UI. The plan feature listing now utilizes checkmarks to denote each feature and the product details are more clearly displayed.
