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/LetterCard.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 import * as m from '$lib/paraglide/messages.js';
 import { timelineDateLabel } from './dateLabel';
+import GlyphLabel from './GlyphLabel.svelte';
 import type { components } from '$lib/generated/api';
 
 type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
@@ -33,8 +34,7 @@ const receiver = $derived(
 		     interpolated into the escaped user title; the title keeps its own
 		     pre-line span for multi-line OCR text (REQ-008/016/021). -->
 		<span class="font-serif text-sm font-bold break-words text-ink">
-			<span aria-hidden="true">✉</span>
-			<span class="sr-only">{m.timeline_letter_glyph_label()}</span>
+			<GlyphLabel glyph="✉" label={m.timeline_letter_glyph_label()} />
 			<span class="whitespace-pre-line">{entry.title}</span>
 		</span>
 	{/if}