feat: the newsletter form submits with htmx and reports failed signups

The newsletter subscription plugin now sits in a form island like the
EXT:form forms: hx-boost posts to the page itself and only
#newsletter-island is swapped (validation errors, confirmation), with the
spinner overlay while the request runs. The id keeps htmx away from other
islands on the same page (both newsletter pages have hidden EXT:form
forms). Without JavaScript the form still posts normally.

The island markup moves from Form/Render.html into Partials/Form/Island.html,
shared by both; the form plugin's partial path is now Partials/ for that.
The comment about static pages is gone with it, static rendering is retired.

When the MyE&P API call fails, the result page shows an error with a link
back to the form instead of the thank-you message.

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

packages/ep_theme/Configuration/TypoScript/Extensions/Form.typoscript

@@ -4,8 +4,8 @@
 
 plugin.tx_form.view {
     templateRootPaths.10 = EXT:ep_theme/Resources/Private/Templates/Form/
-    # Spinner.html for Render.html
-    partialRootPaths.10 = EXT:ep_theme/Resources/Private/Partials/Form/
+    # Form/Island.html for Render.html
+    partialRootPaths.10 = EXT:ep_theme/Resources/Private/Partials/
 }
 
 plugin.tx_form.settings.yamlConfigurations {