Insert Row modal: NULL and default value placeholders look like real user input

## Problem
In the "Insert row" modal, nullable fields with no default show the literal text "NULL" inside the input field. This looks identical to user-typed text, creating ambiguity about whether the field already has a value entered.

Similarly, fields with defaults show "default: active" or "default: 0" as placeholder-style text inside inputs — this could also be mistaken for actual entered values.

## Expected behaviour
- Truly empty / NULL-defaulting fields should show a visually distinct placeholder (e.g., greyed-out italic text, or an empty input)
- Default values could be shown as actual pre-filled values (with the option to clear) or as styled placeholder text that is clearly not a user-entered value
- HTML `placeholder` attribute (shown greyed-out and disappearing on focus) is the standard UX pattern for this

## Steps to reproduce
1. Click "+ New row" on any table with nullable columns
2. Fields like `name` show "NULL" in solid text inside the input
3. It is unclear whether this is a value or a hint