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",

frontend/src/routes/SearchFilterBar.svelte

@@ -281,12 +281,17 @@ $effect(() => {
 					     pages (not the page slice). Stays visible regardless of the toggle
 					     state so it advertises the triage backlog size (issue #668). -->
 					{#if undatedCount > 0}
+						<!-- The bare number "42" reads as meaningless next to the toggle label
+						     (a screen reader would announce "Nur undatierte 42"). The chip carries
+						     a self-describing accessible name ("42 undatierte Dokumente") and its
+						     purely-visual digit is hidden from assistive tech (issue #668, Leonie). -->
 						<span
 							data-testid="undated-count"
+							aria-label={m.docs_filter_undated_count_label({ count: undatedCount })}
 							class="inline-flex min-w-[1.5rem] items-center justify-center rounded-full px-1.5 py-0.5 text-[0.65rem] leading-none tabular-nums {undated
 								? 'bg-primary-fg/20 text-primary-fg'
 								: 'bg-line text-ink-2'}"
-							>{undatedCount}</span
+							><span aria-hidden="true">{undatedCount}</span></span
 						>
 					{/if}
 				</button>