fix(timeline): give the event-card letter count a screen-reader label

The bare "· {count}" spans in both the same-year and cross-year headers
announced as "· 2" with no context. Each now pairs the aria-hidden visible
count with an sr-only "{count} Briefe" via a new Paraglide key
(timeline_cluster_letter_count, present in de/en/es). Fixes review
finding #6 (count half).

Refs #850
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1054,6 +1054,7 @@
 	"timeline_bucket_show_more": "+ {count} weitere Briefe anzeigen",
 	"timeline_bucket_show_less": "Weniger anzeigen",
 	"timeline_letter_glyph_label": "Brief",
+	"timeline_cluster_letter_count": "{count} Briefe",
 	"timeline_tag_chip_label": "Thema",
 	"timeline_layer_historical_suffix": "historisch",
 	"timeline_strip_density_caption": "Monats-Dichte",

frontend/messages/en.json

@@ -1054,6 +1054,7 @@
 	"timeline_bucket_show_more": "+ {count} more letters",
 	"timeline_bucket_show_less": "Show fewer",
 	"timeline_letter_glyph_label": "Letter",
+	"timeline_cluster_letter_count": "{count} letters",
 	"timeline_tag_chip_label": "Topic",
 	"timeline_layer_historical_suffix": "historical",
 	"timeline_strip_density_caption": "Monthly density",

frontend/messages/es.json

@@ -1054,6 +1054,7 @@
 	"timeline_bucket_show_more": "+ {count} cartas más",
 	"timeline_bucket_show_less": "Mostrar menos",
 	"timeline_letter_glyph_label": "Carta",
+	"timeline_cluster_letter_count": "{count} cartas",
 	"timeline_tag_chip_label": "Tema",
 	"timeline_layer_historical_suffix": "histórico",
 	"timeline_strip_density_caption": "Densidad mensual",