feat(timeline): show the root-tag chip on the letter card

LetterCard now renders a TagChip beneath the sender→receiver/date line
whenever the entry carries a rootTagName, mapping rootTagColor to the chip
(neutral when null). Because the chip lives on LetterCard it shows up wherever
a LetterCard does — the global timeline and the expanded YearLetterStrip — with
no per-surface special-casing; a tagless letter shows no chip. A long name
truncates inline so the card never overflows at 320px.

Refs #835
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/LetterCard.svelte

@@ -2,6 +2,7 @@
 import * as m from '$lib/paraglide/messages.js';
 import { timelineDateLabel } from './dateLabel';
 import GlyphLabel from './GlyphLabel.svelte';
+import TagChip from './TagChip.svelte';
 import type { components } from '$lib/generated/api';
 
 type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
@@ -46,4 +47,9 @@ const receiver = $derived(
 			<span data-testid="letter-date"> · {dateLabel}</span>
 		{/if}
 	</span>
+	{#if entry.rootTagName}
+		<!-- The primary root-tag chip sits on its own line beneath the meta line
+		     (#835 §3); absent when the letter has no tag (REQ-005). -->
+		<TagChip name={entry.rootTagName} color={entry.rootTagColor ?? null} />
+	{/if}
 </a>