[gharlan]

Adds a new form system built on the view system. It is meant to replace the old one (DbForm, ConfigForm and the forms built from the core/form/* fragments) without a migration path; the old system is removed before 6.0.

Concept

A form edits a data object. Its properties hold the values in their real types, attributes declare the field types:

final class SystemSettings
{
    #[Choice(
        new Translatable('backend_language'),
        choices: self::locales(...),
        required: true,
        key: 'lang',
    )]
    public string $language = 'en_gb';

    #[ArticlePicker(
        new Translatable('system_setting_start_article_id'),
        required: true,
        key: 'start_article_id',
    )]
    public ?Article $startArticle = null;

    #[Checkbox(new Translatable('system_setting_article_history'))]
    public bool $articleHistory = false; // stored as `article_history`
}

$settings = new SystemSettings();
$storage = new ConfigStorage(Core::CONFIG_NAMESPACE);
$storage->load($settings);

$form = new Form($settings, title: new Translatable('system_settings'));
if ($form->handleRequest()) {
    $storage->save($settings); // or any own save routine reading $settings->startArticle etc.
}

echo $form->render();

• Submitted values are converted into the property types (scalars, backed enums, BcMath\Number, Article, Media, DateTimeImmutable, lists). Invalid values never reach the object; they are kept as raw values and redisplayed with the error.

• The constraints of the field types (required, maxLength, valid e-mail addresses and URLs, min/max of numbers and dates, the offered choices, existing articles/media, allowed media types) are checked on submit; where HTML has an equivalent (required, maxlength, type="email"/type="url", min/max/step), it is set on the input element as well.

• Storages use the property name in snake_case as key ($articleHistory → article_history), unless the field sets an explicit key.

• Each field type declares which property types it supports and every field property needs a default value – both are checked when the class is read.

• Choices and other dynamic arguments are passed as first-class callables (self::locales(...)), labels as Translatable (a lazily resolved translation key, usable in attributes).

• Texts: labels, notes, legends, titles and errors accept string|Renderable – a string is plain text, a Translatable renders like I18n::msg() (basic tags of the message are kept), an Html object passes trusted markup. Choice labels, placeholders and the submit label are plain text (string|Translatable), as they end up in contexts without HTML. Translatable is deliberately not Stringable: render() for HTML, translate() for plain text.

• Rendering: render() for the whole form, or capture() for custom markup – the closure writes the markup inside the form tag, using row()/widget()/label()/rest()/submitButton():

  echo $form->capture(static function (Form $form): void { ?>
      <div class="row">
          <div class="col-md-6"><?= $form->row('sender') ?></div>
          <div class="col-md-6"><?= $form->row('listId') ?></div>
      </div>
      <?= $form->rest() ?>
      <?= $form->submitButton() ?>
  <?php });

  open()/close() are available for form tags that can not be rendered in one place. #[Fieldset] groups fields in the default rendering.

• Embedding: submit() binds given values without the standalone request handling (marker, csrf token), only restricts a form to a subset of its fields – needed for meta info, which will follow in a separate PR.

Field types and widgets

Text (with maxLength), Email, Url (only http/https by default), Number (int, float for approximate values, BcMath\Number for exact decimals if ext-bcmath is available; with min/max/step, also as numeric strings), Textarea (with maxLength), Checkbox, Choice (select, multi-select, radio buttons, checkboxes, with groups), DatePicker, DateTimePicker, TimePicker (with min/max, also relative like today), ArticlePicker and MediaPicker (single and multiple).

The field types only bind the values; the markup comes from widgets (Input, Textarea, SelectInput, ChoiceList, CheckboxInput, ArticleInput, ArticleListInput, MediaInput, MediaListInput) and FieldRow. They are standalone view components, so they can be used without a form as well (e.g. by yform), and their views can be overridden per class via ViewResolver. No fragments and no RexVar widgets are used. The picker widgets still follow the REX_LINKLIST_*/REX_MEDIA_*/REX_MEDIALIST_* element ids the linkmap and media pool scripts rely on.

Users

• The system settings page uses it for both of its forms: the settings with ConfigStorage, the editor settings with their own cookie routine and custom markup.
• The test addon gets a page showing all field types (System → Form), added to the visual tests.

Not included yet

• Meta info on top of the form system (next PR).
• Porting the remaining pages and removing the old form system.