feat(timeline): add the LetterBucket cluster component

Renders one loose-letter cluster for Ereignis/Thema mode (#827): an
"✉ <event> · <n>" header over .lcard.ev cards in Ereignis, a tinted
BucketHeaderChip over chip-suppressed cards in Thema, and a localized
"Weitere Briefe"/"Ohne Thema" header with plain cards for the fallback bucket.

Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/LetterBucket.svelte

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+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import LetterCard from './LetterCard.svelte';
+import BucketHeaderChip from './BucketHeaderChip.svelte';
+import { entryKey } from './entryKey';
+import type { LetterBucket } from './timelineGrouping';
+
+/**
+ * One cluster of loose letters under a header, in Ereignis or Thema mode (#827). The axis-fixed
+ * event/world-band layers are rendered elsewhere — this is only the loose-letter bundling.
+ * Ereignis: a "✉ <event> · <n>" header over `.lcard.ev` cards (REQ-003/014). Thema: a tinted
+ * root-tag header chip over cards whose own tag chip is suppressed (REQ-004/015/017). A bucket
+ * with no title (kind `fallback`) uses the localized "Weitere Briefe"/"Ohne Thema" label and
+ * keeps its letters as plain cards (REQ-006/007).
+ */
+let { bucket, mode }: { bucket: LetterBucket; mode: 'event' | 'thema' } = $props();
+
+const count = $derived(bucket.letters.length);
+const fallbackLabel = $derived(
+	mode === 'event' ? m.timeline_bucket_other_letters() : m.timeline_bucket_no_topic()
+);
+</script>
+
+<section class="my-3" data-testid="letter-bucket" data-bucket-kind={bucket.kind}>
+	<header class="mb-2 flex items-center gap-2">
+		{#if mode === 'thema' && bucket.kind === 'tag'}
+			<BucketHeaderChip name={bucket.title ?? ''} color={bucket.color} />
+		{:else if mode === 'event' && bucket.kind === 'event'}
+			<span class="font-serif text-sm font-bold text-ink">
+				<span aria-hidden="true">✉</span>
+				{bucket.title}
+			</span>
+		{:else}
+			<span class="font-sans text-xs font-semibold text-ink-3">{fallbackLabel}</span>
+		{/if}
+		<span data-testid="bucket-count" class="font-sans text-xs text-ink-3">· {count}</span>
+	</header>
+	<ul class="space-y-2">
+		{#each bucket.letters as letter (entryKey(letter))}
+			<li>
+				{#if mode === 'event'}
+					<LetterCard entry={letter} variant={bucket.kind === 'event' ? 'event' : 'plain'} />
+				{:else}
+					<LetterCard entry={letter} suppressTagChip={true} />
+				{/if}
+			</li>
+		{/each}
+	</ul>
+</section>

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

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+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import * as m from '$lib/paraglide/messages.js';
+import LetterBucket from './LetterBucket.svelte';
+import { makeEntry } from './test-factories';
+import type { LetterBucket as Bucket } from './timelineGrouping';
+
+afterEach(() => cleanup());
+
+const eventBucket: Bucket = {
+	key: 'event:e1',
+	kind: 'event',
+	title: 'Briefe von der Front',
+	color: null,
+	letters: [makeEntry({ documentId: 'a' }), makeEntry({ documentId: 'b' })]
+};
+
+const tagBucket: Bucket = {
+	key: 'tag:t1',
+	kind: 'tag',
+	title: 'Krieg',
+	color: 'sienna',
+	letters: [makeEntry({ documentId: 'c', rootTagName: 'Krieg', rootTagColor: 'sienna' })]
+};
+
+describe('LetterBucket — Ereignis mode (REQ-003/006/014)', () => {
+	it('shows the event title and the cluster count', () => {
+		render(LetterBucket, { bucket: eventBucket, mode: 'event' });
+		expect(document.body.textContent).toContain('Briefe von der Front');
+		expect(document.querySelector('[data-testid="bucket-count"]')?.textContent).toContain('2');
+	});
+
+	it('renders its letters as .lcard.ev event cards (REQ-014)', () => {
+		render(LetterBucket, { bucket: eventBucket, mode: 'event' });
+		expect(document.querySelectorAll('a.lcard.ev')).toHaveLength(2);
+	});
+
+	it('uses the localized "Weitere Briefe" label and plain cards for the fallback bucket (REQ-006)', () => {
+		const fb: Bucket = {
+			key: '__fallback__',
+			kind: 'fallback',
+			color: null,
+			letters: [makeEntry({ documentId: 'x' })]
+		};
+		render(LetterBucket, { bucket: fb, mode: 'event' });
+		expect(document.body.textContent).toContain(m.timeline_bucket_other_letters());
+		// fallback letters are not clustered under a curated event → plain card, never .lcard.ev
+		expect(document.querySelector('a.ev')).toBeNull();
+	});
+});
+
+describe('LetterBucket — Thema mode (REQ-004/007/015/017)', () => {
+	it('renders a tinted bucket-header chip carrying the root-tag name (REQ-015)', () => {
+		render(LetterBucket, { bucket: tagBucket, mode: 'thema' });
+		const chip = document.querySelector('[data-testid="bucket-header-chip"]');
+		expect(chip?.textContent).toContain('Krieg');
+	});
+
+	it('suppresses the per-letter tag chip inside its own root-tag bucket (REQ-017)', () => {
+		render(LetterBucket, { bucket: tagBucket, mode: 'thema' });
+		expect(document.querySelector('[data-testid="tag-chip"]')).toBeNull();
+	});
+
+	it('uses the localized "Ohne Thema" label for the untagged fallback bucket (REQ-007)', () => {
+		const fb: Bucket = {
+			key: '__fallback__',
+			kind: 'fallback',
+			color: null,
+			letters: [makeEntry({ documentId: 'y', rootTagName: undefined })]
+		};
+		render(LetterBucket, { bucket: fb, mode: 'thema' });
+		expect(document.body.textContent).toContain(m.timeline_bucket_no_topic());
+	});
+});