feat: forms on TYPO3 13, loaded into the static pages with htmx

The static export replaced every EXT:form form with a contact notice. The
forms are back, rendered by TYPO3 13 and embedded in the static pages:

- Page type 1704 (/<page>/form) renders only the form plugin of a page,
  inside a .form-island element. Every form posts to that URL
  (renderingOptions.additionalParams), because on www the page URL is a
  static file that never reaches TYPO3. htmx swaps the island in place:
  validation errors, multi-step pages and confirmations. A redirect to an
  htmx request becomes HX-Redirect (HtmxRedirectMiddleware), so Redirect
  finishers send the browser to the thank-you page.
- While a request runs, an overlay with a spinner (as in MyE&P) covers
  the form. Styled inside the island: the static pages load the CSS of
  the TYPO3 10 export.
- The bw_captcha image comes from /<page>/captcha for the same reason.
  Both suffixes are in the PageType enhancer of every site with forms
  (semesterende gets one with default '' to keep its URLs).
- Ported from TYPO3 10: form setup, templates and partials, the MyE&P
  address finisher, the job application finisher with the MyE&P Team
  API client (epTeamApi* in secrets.php), the page URL/referrer finisher
  with ReferrerMiddleware (?ref=), the e-mail layout without the system
  footer, bw_captcha 5 and form_element_linked_checkbox 6. The address
  check forms are dropped.
- German language packs (EXT:form messages): availableLanguages = de,
  fetched on every deploy (typo3:language:update).
- The checkbox partials passed null to VHS' substring, which PHP 8.5
  reports as deprecated.
- DDEV no longer turns deprecations into exceptions, matching
  production: EXT:form's RenderFormValueViewHelper uses null as an array
  offset for empty optional fields (core bug, also in 13.4.37).

The static pages get their placeholders from the exporter in EP/website,
see .ddev/plans/static-export-form-islands.patch (local only).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

config/system/secrets.example.php

@@ -28,6 +28,9 @@ return [
             'myEpApiClientSecret' => '',
             'myEpApiUrlAuthorize' => '',
             'myEpApiUrlToken' => '',
+            // MyE&P Team: job application forms (ApplicationApiFinisher), POST /api/application-forms
+            'epTeamApiBaseUrl' => '',
+            'epTeamApiKey' => '',
             // ep:groups-swiss (cron, daily): availability of the group houses on groups.swiss.
             // Production only; the keys are in the TYPO3 10 scheduler tasks 16-21.
             // 'groupsSwissHouses' => [