refactor(timeline): extract a shared GlyphLabel primitive

The aria-hidden glyph + sr-only label markup was hand-copied in LetterCard
and YearLetterStrip. Extract a small GlyphLabel component and use it at
both sites so the accessibility idiom has a single owner.

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

frontend/src/lib/timeline/YearLetterStrip.svelte

@@ -3,6 +3,7 @@ import * as m from '$lib/paraglide/messages.js';
 import { getLocale } from '$lib/paraglide/runtime.js';
 import { formatTickLabel } from '$lib/shared/utils/monthBuckets';
 import Sparkline from '$lib/shared/primitives/Sparkline.svelte';
+import GlyphLabel from './GlyphLabel.svelte';
 import LetterCard from './LetterCard.svelte';
 import { monthHistogram } from './timelineDensity';
 import type { components } from '$lib/generated/api';
@@ -30,8 +31,7 @@ const dezLabel = $derived(formatTickLabel(`${year}-12`, getLocale()));
 <div class="mx-auto max-w-md rounded-sm border border-line bg-surface p-3 shadow-sm">
 	<div class="flex items-center justify-between gap-3">
 		<span class="font-sans text-sm font-bold text-brand-navy">
-			<span aria-hidden="true">✉</span>
-			<span class="sr-only">{m.timeline_letter_glyph_label()}</span>
+			<GlyphLabel glyph="✉" label={m.timeline_letter_glyph_label()} />
 			{m.timeline_letters_count({ count: letters.length })}
 		</span>
 		<button