Refactor Default Values

Is your feature request related to a problem? Please describe.

Currently default values have two purposes:

1. When creating a new row, those values are prefilled in the corresponding column input fields
2. When a new column is created for a table that already contains some rows, the cells of the newly created column for those rows are filled with the default values. (More detailed: Whenever a cell value is null and has a default value, this value is shown in the table instead)

However, 2. is problematic since this creates problems with both frontend and backend filtering and more.

Describe the solution you'd like

Default values should only be used for the 1. purpose named above.
For the 2. case you could add a new field to the create column modal that allows to fill the cell of this column for all existing rows with this value.

Describe alternatives you've considered

No response

Additional context

Related: #411

Start by tracing how default values are handled in the create-column modal, new-row inputs, existing cells, and frontend/backend filtering. Review related issue #411 and map the current behavior before changing it. Done means defaults only prefill new-row inputs, while filling existing rows is an explicit separate action.