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",

frontend/src/lib/messages.spec.ts

@@ -100,6 +100,14 @@ describe('message key parity', () => {
 		expect(es).toMatchObject({ timeline_tag_chip_label: 'Tema' });
 	});
 
+	// #850 finding #6: the event-card letter count carries an sr-only "{count} Briefe" so the
+	// bare "· 2" never announces to a screen reader without context.
+	it('zeitstrahl cluster letter-count key is present in all locales (#850 finding #6)', () => {
+		expect(de).toHaveProperty('timeline_cluster_letter_count');
+		expect(en).toHaveProperty('timeline_cluster_letter_count');
+		expect(es).toHaveProperty('timeline_cluster_letter_count');
+	});
+
 	// #780 REQ-010: the layer-filter strings are Paraglide keys in every locale.
 	// timeline_filter_trigger (0 active) and timeline_filter_trigger_active ({count},
 	// ≥1 active) are distinct keys so the trigger never reads "Filter (0 aktiv)".

frontend/src/lib/timeline/EventCluster.svelte

@@ -86,7 +86,11 @@ const hiddenCount = $derived(letters.length - CLUSTER_PREVIEW);
 					>{event.title}</span
 				>
 				<span class="block font-sans text-xs text-ink-3">
-					{eventSubtitle} <span data-testid="event-count">· {count}</span>
+					{eventSubtitle}
+					<span data-testid="event-count">
+						<span aria-hidden="true">· {count}</span>
+						<span class="sr-only">{m.timeline_cluster_letter_count({ count })}</span>
+					</span>
 				</span>
 			</span>
 			{#if canEdit}
@@ -111,7 +115,10 @@ const hiddenCount = $derived(letters.length - CLUSTER_PREVIEW);
 				<GlyphLabel glyph="✉" label={m.timeline_letter_glyph_label()} />
 				{title}
 			</span>
-			<span data-testid="event-count" class="font-sans text-xs text-ink-3">· {count}</span>
+			<span data-testid="event-count" class="font-sans text-xs text-ink-3">
+				<span aria-hidden="true">· {count}</span>
+				<span class="sr-only">{m.timeline_cluster_letter_count({ count })}</span>
+			</span>
 		</header>
 	{/if}
 

frontend/src/lib/timeline/EventCluster.svelte.spec.ts

@@ -108,6 +108,16 @@ describe('EventCluster — contained event card (#850)', () => {
 		expect(srOnly?.textContent).toBe(m.timeline_letter_glyph_label());
 	});
 
+	it('gives the letter count an sr-only "{count} Briefe" label so "· 2" is not announced bare (finding #6)', () => {
+		render(EventCluster, { letters: letters(2), title: 'Briefe von der Front' });
+		const count = document.querySelector('[data-testid="event-count"]') as HTMLElement;
+		// the visible "· 2" stays aria-hidden; the sr-only sibling carries the meaning
+		expect(count.querySelector('[aria-hidden="true"]')?.textContent).toContain('· 2');
+		expect(count.querySelector('.sr-only')?.textContent).toBe(
+			m.timeline_cluster_letter_count({ count: 2 })
+		);
+	});
+
 	it('renders an HTML-bearing event title verbatim as text, never as markup (REQ-010)', () => {
 		render(EventCluster, {
 			letters: letters(1),