feat(document): explain auto-generated title under the edit title field (#726)

Adds the FR-TITLE-005 helper line under the title input in DescriptionSection, shown only
on the single-document edit form via a new showTitleHelp prop (off for the new-document and
bulk-edit forms). It is wired to the input with aria-describedby and uses text-ink-3 (WCAG AA
on bg-surface). New Paraglide key form_helper_title_autogenerated in de/en/es. Adds a
component test for the helper + aria wiring and an end-to-end pass: create an auto-titled doc,
edit its date, and see the title follow on the detail page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -56,6 +56,7 @@
 	"form_label_sender": "Absender",
 	"form_label_receivers": "Empfänger",
 	"form_label_title": "Titel",
+	"form_helper_title_autogenerated": "Wird automatisch aus Datum und Ort gebildet — sobald du den Titel änderst, bleibt deine Version erhalten.",
 	"form_label_tags": "Schlagworte",
 	"form_label_content": "Inhalt",
 	"form_placeholder_content": "Kurze Beschreibung des Inhalts…",

frontend/messages/en.json

@@ -56,6 +56,7 @@
 	"form_label_sender": "Sender",
 	"form_label_receivers": "Recipients",
 	"form_label_title": "Title",
+	"form_helper_title_autogenerated": "Generated automatically from the date and place — as soon as you edit the title, your version is kept.",
 	"form_label_tags": "Tags",
 	"form_label_content": "Content",
 	"form_placeholder_content": "Brief description of the content…",

frontend/messages/es.json

@@ -56,6 +56,7 @@
 	"form_label_sender": "Remitente",
 	"form_label_receivers": "Destinatarios",
 	"form_label_title": "Título",
+	"form_helper_title_autogenerated": "Se genera automáticamente a partir de la fecha y el lugar; en cuanto edites el título, se conservará tu versión.",
 	"form_label_tags": "Etiquetas",
 	"form_label_content": "Contenido",
 	"form_placeholder_content": "Breve descripción del contenido…",