fix(documents): give the undated count chip a self-describing a11y name

A screen reader announced the bare number ("Nur undatierte 42"). Add an
aria-label ("42 undatierte Dokumente") via a new i18n key and hide the
purely-visual digit with aria-hidden, so the toggle + count read sensibly.

Refs #668

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/de.json

@@ -101,6 +101,7 @@
 	"docs_list_unknown": "Unbekannt",
 	"docs_group_undated": "Undatiert",
 	"docs_filter_undated_only": "Nur undatierte",
+	"docs_filter_undated_count_label": "{count} undatierte Dokumente",
 	"docs_range_excludes_undated": "Ein Datumsfilter schließt undatierte Dokumente aus, da sie keinem Zeitraum zugeordnet werden können.",
 	"docs_group_unknown": "Unbekannt",
 	"doc_section_who_when": "Wer & Wann",

frontend/messages/en.json

@@ -101,6 +101,7 @@
 	"docs_list_unknown": "Unknown",
 	"docs_group_undated": "Undated",
 	"docs_filter_undated_only": "Undated only",
+	"docs_filter_undated_count_label": "{count} undated documents",
 	"docs_range_excludes_undated": "A date range filter excludes undated documents, because they cannot belong to any time span.",
 	"docs_group_unknown": "Unknown",
 	"doc_section_who_when": "Who & When",

frontend/messages/es.json

@@ -101,6 +101,7 @@
 	"docs_list_unknown": "Desconocido",
 	"docs_group_undated": "Sin fecha",
 	"docs_filter_undated_only": "Solo sin fecha",
+	"docs_filter_undated_count_label": "{count} documentos sin fecha",
 	"docs_range_excludes_undated": "Un filtro de intervalo de fechas excluye los documentos sin fecha, ya que no pueden pertenecer a ningún periodo.",
 	"docs_group_unknown": "Desconocido",
 	"doc_section_who_when": "Quién & Cuándo",