Add 'allowCustomValue' flag to string field combobox

[david-roper]

Summary

Exposes the ComboBox allowCustomValue flag through the form's combobox string field variant, so a form field can accept text that doesn't match any of its options.

Changes

StringFieldComboBox

• Added an optional allowCustomValue?: boolean prop (defaults to false, preserving current behaviour).
• The flag is translated into a value-builder rather than forwarded as-is. This field's items are { label, value } objects, and ComboboxRoot's boolean path commits the raw typed string as the value — which would make onValueChange's item?.value come back undefined. It now passes (inputValue) => ({ label: inputValue, value: inputValue }).
• Fixed the derivation of selected. A custom value is by definition absent from items, so items.find(...) ?? null resolved to null; because the ComboBox is controlled, the input blanked out the instant a custom value was committed. It now falls back to an item built from the value itself when the flag is on.

StringField.stories.tsx

• The ComboBox story's allowCustomValue is now toggleable from the controls panel, with an explicit argTypes entry (docgen inference on the union-typed StringField doesn't reliably produce the boolean control).
• The decorator reads allowCustomValue from its story context. Args passed to <Story args={...} /> inside a decorator override story-level args, so a hardcoded value there would render the control but make it inert.
• Typed this one story as StoryObj<StringFieldComboBoxProps>. Reading args.allowCustomValue off StoryObj<typeof StringField> doesn't typecheck, since those args are the full StringFieldProps union and the input/textarea/password members have no such property. Runtime is unchanged — the meta still renders StringField, which dispatches on variant.

Notes for reviewers

• Consumers can't set this from a form config yet. Form fields are typed by StringFormField from @douglasneuroinformatics/libui-form-types, where combobox shares a union member with radio and select. A form config object literal carrying allowCustomValue will fail excess-property checking until that package splits the combobox variant out and adds the prop. For now the flag only reaches the component when StringFieldComboBox is rendered directly (as the Storybook story does). Upstream change to follow.
• A committed custom value is not a key of options, so any validation schema for the field has to accept arbitrary strings rather than a fixed enum. This is documented in the prop's JSDoc.

Testing

• tsc --noEmit and eslint clean.
• Existing ComboBox and Form suites pass (30 tests).
• No new tests added. The selected fallback in particular is worth covering — it's the kind of thing a future refactor could quietly undo — so a spec that types a custom value and asserts the input retains it after re-render would be a good follow-up.

[github-actions]

🎉 This PR is included in version 6.14.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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