Add field descriptions and defaults

[priethor]

What

Closes #122.

Adds Field settings for custom fields. Editors can add a description, see help from table headers and row properties, and set defaults for supported field types so new rows start with the expected value.

Why

Fields sometimes need a little more context than a name, and repeated values should not have to be filled in by hand every time someone creates a row.

How

• Applies defaults only when a row is created.
• Leaves existing rows alone.
• Lets explicit row-creation values beat field defaults.
• Keeps defaults type-aware, including today-based date defaults and select/multiselect choices.
• Copies descriptions and defaults when duplicating fields, clears defaults on type changes, and cleans up select defaults when choices change.

Followups

• The column header actions already do too much; the next step should be to refactor them.
• With the new menu item to edit the field settings, some field types have more than one menu to customize them (selects have one to set the options/choices, numbers have a different one to customize how it's displayed). After or alongside the refactor, we should combine all field settings into a single menu option.

Testing Instructions

1. Create a collection with text, number, date, checkbox, select, and multiselect fields.
2. Open a field menu, choose Field settings, add a description, and set a default.
3. Check that the description appears from the help icon in the table header and row properties.
4. Create a new row and confirm the default values are filled in.
5. Change a default and confirm existing rows do not change.
6. Edit select choices and confirm defaults update or clear when their choice is removed.

Screen recording

Grabacion.de.pantalla.2026-05-26.a.las.17.36.11.mov